Chaire Mercier 2020-2021

An established maxim in metaphysics

Conceivability, Possibility, and Hume’s Other Principle

Can we think about the absolutely impossible – that which obtains in no possible circumstance whatsoever, like a logical inconsistency?
A venerable tradition has it that we can’t. Moritz Schlick, for instance, claimed in Positivismus und Realismus that, while the merely practically impossible is still conceivable, the logically impossible is just unthinkable. An alternative tradition, however, runs through Western thought. In the Science of Logic, for instance, Hegel complained against “one of the fundamental prejudices of logic as hitherto understood”, namely that “the contradictory cannot be imagined or thought”. (Hegel [1831]: 430)

The venerable tradition is eminently represented by Hume’s Other Principle from the Treatise (I, ii, 2): Nothing we imagine is absolutely impossible. (I would have liked to call this, ‘Hume’s Principle’, but unfortunately the name was already taken.) Arguments from imaginability to possibility are important in modal epistemology – the branch of epistemology that aims at answering the question: How do we know that something is possible, or necessary, or impossible?

I will argue for the alternative tradition, against the venerable tradition. Several authors have already attacked Hume’s Other Principle. In this lecture, I will join them, hopefully with a fresh strategy: I will present an argument by cases against the Principle, based on different ways of understanding the notion of mental representation.

Leçons suivantes : "The Topics of Thought"

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Intentionality is a feature of some mental states: that of being about, that is, directed towards, objects, situations, states of affairs. Propositional or de dicto intentional states are states having propositions as their contents. These are recorded linguistically by verbs taking sentential complements and expressing attitudes towards said contents, such as ‘believes (that)’, ‘knows (that)’, ‘imagines (that)’, ‘supposes (that)’, ‘is informed (that)’. One may use the generic term ‘thought’ as a cover-all for such intentional states.

In these lectures, I will talk about thoughts, so understood, and what they are about: their topics, as I will say. I will present a new framework for the logic of thought – a unified way of replying to the question: given that one thinks (believes, knows, etc.) something, what else must one think (believe, know, etc.), as a matter of logic? Under which logical operations is one’s thought closed?

The foundations of a logic of intentional states must lie in a general theory of propositions. What Qs one must think, as a matter of logical necessity, because one thinks that P, must depend on the contents of P and Q. Two-component semantics is a theory of propositional content, based on the insight that propositions must feature two irreducible components: (1) truth conditions, and (2) topics. Whereas (1) is familiar, (2) will be introduced and explained in some detail.

Two-component semantics is hyperintensional: it individuates contents in a more fine-grained way than standard intensional or possible worlds semantics. But possible worlds semantics has been a 20th Century philosophical success story: it has been used extensively to analyze the aforementioned notions – knowledge, belief, information – and more. These lectures will also discuss, thus, a number of problems for such applications of possible world semantics.