St. Thomas answers the ancient problem of “the one and the many,” namely the question of how all things can be at once similar and distinct, through a multilayered theoretical metaphysic. The foundation of this structure is his distinction between essence and existence, elaborating on its identification by Aristotle. St. Thomas recognized that being or existence is that which connects all things that truly exist, that are real rather than purely virtual in the mind or in potency, and that being thus makes all real things similar to one another. This interconnected similarity is analogical, in the sense that all real things share being but in distinct ways; this analogical predication of being is through proper proportionality, by which the predication is literal rather than merely metaphorical and indicates an authentic but diversified sharing in the same principle. Taking this further, St. Thomas realized that no created thing possesses being by essence, as necessary to its definition, but rather receives its being from another, via temporal causality and through the ultimate creativity and sustenance of God, who is himself the principle, source and infinite fullness of Being in whom all created things participate for their existence. God is thus the sufficient reason for why there is something rather than nothing, since no creature can account for its own being by itself.

He then specified that created beings are not only similar but also dissimilar, and that this “one and many” can be so through the existence of two really distinct, irreducible metaphysical co-principles in each substance and making it to be both individually subsistent and connected to all other beings. These principles are its singular act of existence, namely its unique participation in being, and its essence, which is its nature, a kind of filter limiting the ways by which the fullness of being may be expressed in it and connecting it to other beings within a particular kind or species of being, reducing the infinity of being into a specific structure. This essential nature is a reflection of and participation in the archetypal image or type for it in the mind of God according to which it is created and to which it strives as a final cause by its self-expressive action, and in this way all things are created through God’s exemplary beauty.

Essence is further clarified by form, with essence thus signifying the species of being by which an individual is connected to others like it and each particular instantiation of that form expressing only certain limited elements of its species through the spatial-temporal individuation of matter. Form is a deeper explanation of essence, articulating that essence on its own cannot account for individual beings but only for the groups or kinds of species which participate in being, with each formal species being distinct from one another but similar in themselves and related within overarching genera. For material beings, those individuals who do not fully exhaust the potentialities of their specific form in themselves (as do angels) but only express certain aspects of it, matter is the metaphysical co-principle which individuates and instantiates the quality of form through a particular extension of being within a quantified spatial and temporal presence. This limitation is accomplished by the composition of material parts which are activated, organized and united according to a specific nature by form, with the dissimilar parts being given intrinsic, rather than merely extrinsic and aggregate, unity and identity as a formal substance.

Form/matter is essential for explaining how material beings can each participate in a shared species while still being individual; without form, material beings would be radically individual without any pattern shared within groups and would also lack the intrinsic unity which organizes matter into a particular kind of individual, while without matter, form is purely abstract and universal, lacking individuality accomplished through the limited potentialities in matter which are activated by form. However, beings which are pure form (realized by their act of existence), such as angels, or which can exist formally apart from matter (even if not constituting a true or full substance in that state), such as humans, are either distinct species, with each individual exhausting the full range of its form, as with angels, or their form has become individuated by the commensuration of the form to the body during life and thus remains individual even when separated from it.

In general, modern philosophy tends to dispute the reality of form; despite the scientific recognition that things are one and many, species and individuals, many advocates of science dispute this formal similarity and thus see all things as mere aggregates of material parts possessing only extrinsic rather than intrinsic unity and thus not constituting true individuals but only a kind of stream of phenomena. The particular understanding of this materialist and reductionist philosophy varies, with some emphasizing the connectedness of things (Whitehead) and others the indistinguishability of things (Bergson), but they are all united by their denial of form and emphasis on matter and thus of existence as in a constant state of flux and particulate recombination without any underlying constancy or similarity beyond subjective experience or atomistic reductionism. Nevertheless, without a recognition of matter as mere potentiality, with prime matter being nonexistent without the activation of form, modern philosophers inadvertently deny the reality of matter itself and become pure formalists, eventually leading to the assignation of rational order in reality to pure subjective experience (Kantian phenomenology). Like all modern philosophies, it is thus a rejection of common human experience and sense, replaced with a more fantastical conception of reality whose acceptance signifies “enlightenment”.

In a sense, other than the Thomistic recognition of Being as the most primary principle and truth of all things, its theory of act/potency is arguably its most important and universal, applying to all its layers of metaphysical explanation for the one and the many and even for the existence of God. Ultimately, act is synonymous with being, since to be means to be in act, to be active, self-expressive and self-revealing through action and to be connected to all other beings, standing out from the nothingness of inactivity; in act, being is thus not a purely static and unchanging reality underlying change (Descartes) but is its dynamic source and integral foundation. Through act/potency, the act of existence can be said to function as the actuality of essence, making it to truly exist and seek its natural fulfillment by action; form functions as the actuality of matter, activating its potencies and thus expressing itself through matter as its organizing principle, as well as the source of life in living beings, with death being a kind of deactivation, the dispersal of matter into constituent but subsequently dead parts without the overarching unification of a substantial form; and substance functions as the active basis for the myriad changes or modes of being which its nature can perform or undergo through its accidents, themselves being the substance’s range of potencies. Finally, since all created beings are a metaphysical composition of the co-principles of act and potency in various ways, with no creature possessing being intrinsically but only by participation, contingently capable of coming into and out of existence, there must be a source who is fully active, with its full nature being completely and eternally expressed within itself without any potency, and this source is God, who gives act to all things that are in act via their participation in his creative Being.

This theory is uniquely Aristotelian, taken up and expanded upon by St. Thomas, and constitutes one of their greatest contributions to philosophy. Rival theories, both ancient and modern, tend to emphasize either act or potency exclusively. As with form/matter, while the ancients tended to see true being as only act, with change or potency being illusory, modern philosophers tend to see all things as in constant flux and so they deny the dynamic composition of the actuation of potency as accounting for change. Instead, since science can only measure that which is real, advocates of scientism reject the idea of potency, seeing only things which are already in act as being truly real; for this reason, they reduce substances to purely material aggregate parts in an atomistic conception, being unable to identify prime matter itself (since it is pure potency), and believe that these parts are in a continual state of flux and reconfiguration which constitutes the apparent formal unity of things on the level of our experience and apprehension without any principle of continuity. However, this ultimately contradicts science itself, which seeks to define things according to their specific natures and thus based on their potencies for different modes of being; from this, science describes how things may or may not act and predicts the results of actions, but this requires the actuation of potency in a real connectedness of action and so cannot be properly understood without the theory of act/potency.

The Thomistic distinction between substance and accidents is in a sense the surface or top layer of its explanation of the one and the many, explaining exactly how things can be not only specific and individual at once, but how they can remain the same while also changing over time. This is accomplished by the idea of substance as an individual, particular unity of form and matter, and so also of an act of existence with a natural essence participating in being. A substance is thus a real, individual being composed of both an integral, consistent identity as well as the capacity to express its essential participation and exemplification of being through temporal change. While substantial unity is that which enables a being to remain the same over time, this consistency is not static, despite the erroneous understandings of Descartes, Whitehead, Hume and others; rather, it is the dynamic capacity for an individual being to take on multiple, distinct modes of existence while retaining its identity. Substance is thus a kind of power or voice echoing throughout a temporal sequence, uniting them as a single symphony. Accidents function as the range of non-essential changes which a substance may undergo without losing its substantial identity, the diverse modes of existence which the substance can experience and by which it can express its particular and individual act of existence with other substances. These accidents, as the potentialities of a substance, are based in its matter and are limited by its formal nature which provides it with the restrictive filter or capacities, like essence for existence, of the modes of being it can take on. To go beyond this range constitutes a substantial change, through which the substance is converted into another substance and its identity is destroyed.

The theory of substance/accidents is essential for a full and authentic understanding of reality. Without it, life becomes either an unchanging substantial mass without variety or self-expression (with only spiritual beings, like God and angels, being pure substances without accidents while remaining individual), or purely accidental without any identity “standing beneath” the changes which happen to it and thus all individuality and interconnectedness is lost. The consequences of the denial of this theory are often lost on those who reject it, but through logical deduction it can easily lead to grave consequences, as can the denial of the other Thomistic metaphysical schemes which answer the question of the one and the many.

Goodness, along with unity, activity, truth and beauty, is an example of a transcendental property of being, a specific aspect of the infinity of being which is distinguished and typified by its self-expression in created beings, while in God it is one with his Being. As such, goodness is a property exhibited by all real things which participate in being. Although convertible with the other transcendental properties, it is conceptually distinguished as expressing the intrinsic fittingness or desirability of being, explicating the nature of being as the ultimate source of all perfections in things. It is thus the inherent orientation of being toward perfecting or fulfilling creatures who participate in being, accomplished through their greater assimilation of being in accordance with their nature and proper operations. However, St. Thomas made the distinction from Aristotle that this goodness is not merely a subjective imposition or disposition in creatures as desiring their own fulfillment, but is rather an intrinsic element of being itself, thus establishing goodness as fully objective, real and universal. As a transcendental property of being, goodness can be predicated of all things which are real and thus participate in being; while this goodness may not always be explicit, it can be deduced from the mere fact that something exists, since being itself is good and thus even the demons are intrinsically good due to God sustaining them in existence and affording to them the goods of their natural essence.

Goodness, like being itself, is both analogous and hierarchical, with different beings sharing in goodness to varying degrees and in diverse ways throughout the universe; in this way, created being as a whole shows forth the goodness of being in a harmonious, symphonic and diverse way, with unity rather than uniformity. Since God is the source and sufficient reason for all being as the infinity of being which is participated in by creatures, he is infinitely good and is goodness itself. Through his creative intentionality, God thus imbues all created good with desirability as a reflection of the goodness of his own being, although in him, due to his divine simplicity, the good is identical with his essence, as are the other transcendental properties distinguishable by the human intellect. Privation or evil as such is not a true rival of goodness but is rather its absence, not based in being but in non-being and representing a lack of goodness proper to the nature of a particular thing; as infinite goodness itself, God has no privation.

The disputation of goodness as an objective, metaphysical transcendental property of being by many modern philosophers suffices to reduce goodness to the purely subjective and so to make being not that which all things desire for the fulfillment of their natures but simply as the imposition of preference onto an ultimately neutral reality, as in the thought of Kant, Rousseau and Nietzsche. Not only does this relegate all moral judgments, as well as aesthetic judgments since beauty requires goodness for its transcendental synthesis, to subjective taste or opinion, but it also contradicts the Thomistic and classical understanding of evil as a privation of being, a lack of the fullness of being afforded to each essence through its act of existence and satisfying the finality to which it strives; evil instead becomes bad conditioning at best and mere arbitrary whim at worst.

Unity is also a transcendental property of being. It consists in the cohesion and substantiality by which a thing has a recognizable identity. In this way, unity is distinguished between the extrinsic and the intrinsic, with the former including only an aggregate or accidental unity represented by those beings which, while identifiable in a conglomerate way by the human intellect, are not true substances as such, like rocks, sand, etc. These things are superficially distinct, but in essence they are only composed of parts which are identical to one another, such as a rock that is merely an aggregate of a certain mineral, and so terms such as “rock” are more so attributed to the accidental form of the thing, giving it its specific coloration, size, etc., than any identifiable, substantial unity. On the other hand, those beings which can be clearly identified, such as humans, other animals, molecules, etc., possess an essential form which provides a unity amongst contrary parts and interdependent metaphysical co-principles. Without this substantial unity, nothing would be identifiable above the level of its smallest parts; this is the claim of reductionism, which ascribes the attributes and changes in material things to the merely superficial recombinations of smaller parts. This view, however, is ultimately untenable, both because it contradicts human experience and common sense which clearly identify substances and distinguish them from aggregates, and because reductionists can never find the lowest level of part but must either end with some ultimate, irreducible substance, as with atomism, or assert that reality is only an illusion made up of nothing or at least of something that cannot be known, as with many modern and Eastern philosophies. As the infinity of being, God is unified to the greatest possible extent, with a personal identity that is undivided and exhibited to the highest degree in the Trinity.

As one of the four forms of causality identified by Aristotle and St. Thomas, efficient causality is one of two causes which is “external,” alongside final causality, helping to explain the changes and relations between real beings as known through experience. Specifically, efficient causality is the innate propensity for and capability of all real beings to impart existence to other beings, through the “generosity of being.” Rather than a series of disconnected, probabilistic patterns, as with empiricist and scientific predictability theories of causality, efficient causality asserts a genuine connection between things on the level of change, with creatures having been given the power by God, through their act of existence, to share their participated being and perfections with other creatures, in necessary, contingent and systematic ways. Significantly, St. Thomas recognized that efficient causality is simultaneous with its effect, not representing two distinct actions but rather one movement from cause to effect, and that the change or imparting of new being takes place entirely within the effect, so that God, who is the Prime Mover and ultimate efficient cause of reality as the exemplar of its participation in being, is not changed within himself by his creative acts but only creation within itself, as the effect, is truly changed. God thus grants to creatures a share in and reflection of his power of efficient causality, within the limitations of their specific natures and the final causality imparted by them.

As free agents, humans also have the added element of moral responsibility, by which we are called, by the first principle of “do good/avoid evil,” to cause effects in things and ourselves which are in accordance with the natural law and right reason, as reflections of God’s Providence, and to prioritize our actions within the hierarchy of being. Final causality, as the other extrinsic cause within the fourfold Aristotelian causality, is directly connected to efficient causality. It is an expression of the God-given specific nature of a thing which limits its range of efficient causal powers, analogous to essence limiting existence, and as such it acts as a “principle of selection” for efficient causality, determining both why one change occurs over another and why each thing possesses or can possess the perfections proper to its nature. Final causality is thus the active movement of a thing from potency to act towards an end, working with efficient causality for the sake of its perfection and distributing perfecting being to others in its systematic environmental relations.

Final and efficient causes are related to and expressive of the interior forms of Aristotelian causality, namely the formal and material, with formal causality determining the form and thus the substantial identity or actuality of a thing, and material causality determining the matter and thus the range of accidents and potencies available to a thing. The formal cause is related to efficient causality as providing the guidelines for its final causality through its formal nature while also functioning as the principle of perdurance between accidental changes, while material causality is related to efficient causality by providing the individual potencies which it can activate, both within itself and its effects, according to its final causality, and serving as the principle of perdurance in substantial changes. All of these kinds of causality depend upon Intelligence as their ultimate cause precisely because nothing is necessary in itself, providing its own sufficient reason, but rather participates in the fullness of being who is God, and thus for the existence and essential natures of all things, from which their causal power derives, they depend upon God. Further, it can be said that all things function according to an “idea” within their nature, according to which their range of possibilities are activated, and thus God is the highest principle of selection as shining the exemplary light of form onto beings from his divine intellect.

According to St. Thomas, there are two kinds of change in beings: substantial and accidental. Substantial change is the transformation of a substantial being from one essential form to another, through the efficient power of some cause. While accidental change accounts for those changes which remain in a thing and do not change its fundamental identity, substantial change destroys the unity of a thing and either breaks it down into its constituent parts or incorporates it into the being of another thing. The primary difficulty with this concept is the intrinsic connectedness between the two substances within a causal chain and what acts as their principle of perdurance. St. Thomas accounted for this with his concept of prime matter, specified into the individual or particularized matter possessed by a singular being, with prime matter being the potencies of a composite substance and its individuation through extension in space and time. Prime matter thus provides the range of possible changes available to a substance, through the particular matter unified by its form, and accounts for its accidental changes. However, in substantial change, prime matter is the principle of perdurance, with the material parts of a substance moving from unity under one form to unity under another at the beginning and end of the causal sequence, accomplished through efficient and final causality. While empiricists and reductionists deny substantial change, and Kantian phenomenologists or nominalists deny our ability to know about it, it is a necessary concept for explaining the changes humans experience on a daily basis, including some of the most significant such as death, and accounts both for our intuitive recognition of causal connections between substances in a chain as well as the fact that a substance is no longer the same after being incorporated into another substance or breaking down into its parts. However, within the more general notion of substance, substantial change also indicates that real substantial unities do exist and thus refutes the claims of reductionism.