One of the leading Thomist thinkers today, Edward Feser, gives a fine outline of key Lockean ideas and the tensions within Locke’s own thought. The book is concise and well-written. We will begin with a survey of medieval scholastic thought, for this is Locke’s foil.
The Form of a thing is the organizational structure of a thing. It is not reducible to the sum of its parts. For Aristotle it exists “in” the thing or at least gives structure to it. A Substance: it is an independently existing thing. Its attributes or accidents cannot exist apart from it.
For Thomas Aquinas a Right (jus) is a right ordering in a social context. It is objective. It wasn’t originally thought of as a moral claim inhering *in* a subject.
Key idea: Locke held many similar aims with the scholastics: both sought to prove the existence of God, personal immortality, and natural rights. On the other hand, Locke rejects almost all of the key scholastic terms such as essence, quality, causation, etc.
Against innate ideas. An idea is the object of understanding (Locke E.2.1.2). All our ideas come from sensation or reflection (our observing mental images). A simple idea is one that has a uniform appearance. A complex idea is composed of simple ideas.
Locke’s substance: combination of simple ideas representing a particular thing capable of subsisting by itself. A mode is something that cannot subsist by itself. Locke held to a “substratum” underlying all our ideas (E. 2.23.1ff). Locke understands there to be both real essences and nominal essences (i.e., complex ideas). A real essence is the internal constitution of a thing (E 3.3.15).
Problem for post-Lockean nominalists: there are mind-independent cases of “resemblances” between entities. Resemblance is a universal. Two particulars agree because they have a “property” in common. The German mathematician Gottlob Frege delivers the killing blow: if the meaning of our words were identical to subjective entities like ideas, which only we can access, then communication is impossible.
The Second Treatise of Government
Law of Nature. Principle of self-ownership: nobody has a right to but himself. This is Locke’s famous view of “self-ownership.” It’s not entirely the same as what modern libertarians teach. We don’t have the right to suicide, for that would violate God’s own property-rights. Similar arguments could be made against prostitution, pornography, and illicit sex (and other libertarian talking points).
The problem: given Locke’s strict conditions for knowledge, are there any knowable sanctions for non-compliance with the law of nature? Another problem is that in “Toleration” and “Second Treatise” Locke seems to see humans as having identifiable natures. His “Essay” rules out such a claim (E 3.6.37).