1.1 From Plato to Aristotle
The earliest development of a theory about formal causation is Plato’s theory of ideas, and his theory is without doubt an important precursor to Aristotle’s theory of the formal cause. For Plato, ideas or forms are transcendent, eternal, and unchangeable causes of both sensible objects and of our knowledge about such objects (the classical overview is Ross 1951). For instance, a beautiful object is beautiful because it ‘participates’ in the idea Beauty, and because of this participation it is also recognisable as something beautiful (Euthd. 6d–e). Plato uses a varied terminology, but his preferred terms are idea and eidos. The Greek term idea is often translated as ‘idea’, but it can also be translated, among others, as form, outward appearance, kind or sort, nature, plan, or ideal. In contrast, the term eidos is, in this context, standardly translated as ‘form’, but it can also be translated as figure, shape, kind, nature, or essence. Sometimes Plato also speaks of a paradeigma, that is, a pattern, model, or example. We will use the term idea for a transcendent form as postulated by Plato.
Unfortunately, Plato’s theory of ideas suffers from well-known problems. Some of these were raised already in Plato’s dialogues (for instance in Parmenides and the Sophist), and more forcefully and systematically by Aristotle (especially in Metaph. I.9 and XIII.4–5, and in the early work On Ideas; for an edition of the latter work, see Fine 1993). First, there are reasons for doubting the coherence of any assumption of transcendent ideas, and, as argued by Aristotle, even more so of Plato’s world of ideas. Moreover, transcendent forms seem not to solve the problems that they were meant to solve, or the solutions were thought to be independent from the transcendence of the forms. Hence, they seem to unnecessarily multiply existing things (including the world of ideas), apparently in violation of Ockham’s razor, which tells us not to multiply entities beyond necessity.
Second, even granting the existence of ideas, there is the epistemological problem of how we can attain knowledge of these forms. For Plato, our senses and experience cannot give us such knowledge: this can only be attained by thought (dianoia) and intuition (noêsis). And even further, as argued in the Meno and Phaedo, this process of thought is possible because the immortal soul has a recollection (anamnêsis) of the forms from the time before the soul had a body. In the Phaedrus (246b–248e), Plato even introduces the famous chariot allegory, picturing the tripartite immortal soul as a feathered chariot drawn by two horses guided by the intuiting mind (nous) as the charioteer, following Zeus and the other gods around on their cruise around the super-heavenly place of the transcendent ideas. Those that have a strong mind as a charioteer that is able to control the horses (the passions of the soul) will get a good glimpse of the ideas and will, when reborn in a body, become philosophers that long to remember what their souls have seen before they got incarnated. Even if we abstract from its poetical embellishment, such a non-empirical and outright mystical epistemology is today very much in a state of disgrace.
Third, there is the issue of the causal efficacy of the ideas. They are for Plato abstract entities, and are completely separate from material entities (see for instance Prm. 130a–134e)—the material entities only mirror these ideas imperfectly. Because of this strict separation, it is hard to see how the ideas could, under any understanding, be actual causes of the material entities.
Fourth, Plato’s theory of ideas has been argued to give rise to a logical vicious regress, called the ‘Third man argument’ (both in Plato’s Parmenides and by Aristotle, see the references above). For instance, Socrates is a man because he participates in the idea Man. But according to Plato’s principle of self-predication of ideas, this idea Man is also a man, so we need, in parallel with the first step, another idea Man3, in which both Socrates and the idea Man participate, and thus there will be three men. As Man3 is also a man, we get into a vicious regress, where each new step in the regress is needed in order to account for why the man in the previous step is a man. However, it is a contested question whether Plato is in fact vulnerable to this regress (see for instance Aristotle, Soph. El. 22, 178b37–179a11 and Fine 1993: 225–41).
Aristotle intends for his theory of form (eidos or morphê) to be invulnerable to all of these problems. In order to do so, he rejects Plato’s world of ideas as something separate from the world of material things, and instead posits that the forms are dependent upon the particular instances of the form—somewhat misleadingly, the forms are said to be ‘in’ the material things. For Aristotle, too, forms have a number of important roles, especially in ontology, in epistemology, and in philosophy of science.
We can distinguish at least two phases in Aristotle’s thinking about formal causation, in which he develops two distinct theories of formal causation which are logically independent, yet mutually compatible. (Ferejohn 2013 argues for three phases: he adds an intermediate empirical phase where Aristotle rejects formal causation and instead favours final causation.) The first, and presumably chronologically earlier, phase is non-hylomorphic, and especially represented in the Posterior Analytics. In this phase, Aristotle’s position is reminiscent of Plato’s. Both the terminology and the explanatory roles of forms are similar, although Aristotle clearly rejects Plato’s view of forms as transcendent ideas that are existentially independent from the this-worldly particulars that instantiate these forms (APo I.11, 77a5–9; I.22, 83a33–5). Aside from this difference, Aristotle agrees with Plato on many points. For both, for example, the objects of scientific knowledge (epistemê) are universals (katholou, cf. APo I.24). However, the specifics of Aristotle’s theory of scientific knowledge are still different from Plato’s. Universals, on Aristotle’s conception, captures that which holds universally or generally for all instances (kata panta) of a kind. For instance, it holds at any time that all humans are living beings. This connection between being a human and being a living being requires no extrinsic ground, but holds in virtue of the being (ousia) or real definition (logos) of the universal Human. Other examples that Aristotle gives are the connection between being a line and being straight, and between being a prime number and being a number. In Aristotle’s terminology, these connections are per se (kath’ hauto), or one might say some sort of relation of essence. In sum, we here find an early conception of universals and essences.
In the second and presumably later phase, formal causation was integrated with Aristotle’s theory of hylomorphism. This theory holds that all material substances are composed of two distinct components: its matter and its form. Hylomorphism also neatly integrates with the account of change in Aristotle’s Physics, which postulates an underlying substratum (hypokeimenon) in order to rebut Parmenides’s denial of change. First, Aristotle distinguishes between accidental and substantial change. Accidental changes are cases where a substance changes one of its accidental properties, that is, properties in categories other than that of substance. For instance, Socrates changes from being in the market square to being at home (change in place), or Socrates changes from sitting to standing (change in position), or Socrates changes from being white to being tanned (change in quality). In these cases, there is something relatively stable—the substance, Socrates—which remains throughout the change. For substantial change, for example, for Socrates’s ceasing to be after drinking hemlock, Aristotle clearly needs a different answer, since in such cases the substance cannot be the underlying substratum. Hylomorphism provides him with an answer: what remains the same throughout substantial change is the matter of a substance. When Socrates dies, his matter becomes the matter of a new substance: a corpse.
In the Metaphysics, hylomorphism is applied to a different question: what is being (ti to on)? Taking up several points from the Categories, Aristotle says at the beginning of Metaphysics VII that being is said of many things (substance, quality, quantity, place, etc.). But since substances are the bearers of properties, substances are thought to be primary, while the entities which fall under the other categories are in some way dependent upon substances (for discussion on the dependence relation involved, see Corkum 2013; Koslicki 2013; Schnieder and Werner 2021, this volume). But in Metaphysics VII Aristotle also says that substance is said of many things: the essence or form, the universal, the genus, and the matter (1028b34–1029a1). The rest of the book is concerned, then, with whic...