In every material thing or substance there are two distinguishable constitutive principles. [Footnote: The word 'element might suggest chemical element and so be misleading. By calling matter and form 'principles,' Aquinas means that they are the primary co-constituents of a material thing. The word is obviously not being used in the sense of a logical principle; nor does it refer to observable chemical elements. Matter and form are 'principles of being' (principia entis); they are not themselves physical entities.] One of these he called 'substantial form.' In the case of an oak tree, for example, the substantial form, corresponding to Aristotle's 'entelechy' is the determining principle which makes the oak tree what it is. This form must not, of course, be confused with the outward shape or figure of the tree: it is an immanent constitutive principle of activity which makes the oak tree an oak tree, stamping it, as it were, as this particular kind of organism and determining it to act as a totality in certain specified ways. But what is it that the substantial form of the oak tree 'informs' or determines? We might be inclined to answer that it is the matter of the tree, meaning by this the visible material which can be chemically analyzed. But ultimately, Aquinas thought, we must arrive at the concept of a purely indeterminate potential element which has no definite form of its own and no definite characteristics. This he called 'first matter' (materia prima). Visible matter, secondary matter, is already informed and possesses determinate characteristics; but if we think away all forms and all determinate characteristics we arrive at the notion of a purely indeterminate constitutive principle which is capable of existing successively in union with an indefinite multiplicity of forms.
When the oak tree perishes, its substantial form disappears, relapsing into the potentiality of matter, but the first matter of the tree does not disappear. It does not, and indeed cannot, exist by itself; for any existent material substance is something definite and determinate. When the oak tree perishes, the matter immediately exists under another form or forms. When a human being dies and his body disintegrates, the matter is at once informed by other forms. But there is continuity, and it is the first matter which is the element of continuity.
According to Aquinas, therefore, every material thing or substance is composed of substantial form and first matter. Neither principle is itself a thing or substance; the two together are the component principles of a substance. And it is only of the substance that we can properly say that it exists. 'Matter cannot be said to be; it is the substance itself which exists' (C. G. 2,54).