Robert Pasnau, in his Metaphysical Themes 1274-1671, draws attention to two ways in which we find Hobbes talking about substance. One is found in De Corpore, among other texts. On this, “there is no room for metaphysical entities like the thin substance and its inhering accidents” (Pasnau 117). Indeed Hobbes wrote against Bramhall that nothing could be compounded of substance and accident. However we also find Hobbes talking about substance in the earlier Third Objections, and there he appears more open to a substance-accident distinction and to accidents as “metaphysical parts” of things. Hobbes says there that “all philosophers distinguish the subject from its faculties and acts” and that “even the old Peripatetics taught clearly enough that substance is not perceived by the senses but is inferred by reasoning” (Pasnau 137). This suggests a picture on which we perceive accidents and infer the existence of the (underlying) body, contrary to the first picture. So how do the two ways of talking about substance fit together?
Despite acknowledging the possibility that Hobbes might have changed his mind about this issue between the Third Objections and the later texts, Pasnau prefers to understand Hobbes as engaging in the Third Objections in an ad hominem argument against Descartes, relying on premises that have been accepted by others. However, I wonder whether there is a story to tell about Hobbes changing his mind here, one that is more that just a blunt statement that he did so.
Elsewhere ,  I have argued that Hobbes changed his mind over time about materialism. Early on (e.g., in the Third Objections) he thought that incorporeal things could not be imagined, though they could be conceived of in another way. Only later on did he come to believe that those incorporeal things actually did not exist. That is, he moved over time from a (weaker) unimaginably claim to a (stronger) non-existence claim.
I wonder whether we might not say the same sort of thing about Hobbes’s views about substance. In Objection IX of the Third Objections, we find Hobbes saying:
I have frequently remarked above that there is no idea of God or of the soul. I now add that there is no idea of substance, for substance (given that it is matter subject to accidents and changes), is something concluded to solely by a process of reasoning; nevertheless, it is not conceived nor does it display any idea to us.
This puts substance on an equal footing with alleged incorporeal beings, in the early text: we cannot imagine (have ideas or images) of them, but we can reason that they exist (and in a way conceive of them). In the later texts, Hobbes came to argue that there simply are no incorporeal things, only bodies. In a similar way, we might see him as coming to think that there are no substances that underly accidents: again, there are only bodies. Thus we do not simply find an unexplained change of mind in Hobbes’s views about substance, but a change of mind that fits a broader pattern.
(This is, I suppose, the second in a very occasional series on Pasnau’s book. Previous post here.)