 Assuming that such a priori knowledge exists in some way or other, much recent philosophy has occupied itself with the issue of how it is possible. One traditional (“rationalist”) view is that the mind is equipped with a special capacity to perceive truths through the examination of the relations between pure ideas or concepts, and that the truths reached through the correct operation of this capacity count as known a priori. (See, e.g., Leibniz's “Discours de Métaphysique”, §§23 ff.; Russell 1912, p. 105; BonJour 1998 is a very recent example of a view of this sort.) An opposing traditional (“empiricist”) view is that there is no reason to postulate that capacity, or even that there are reasons not to postulate it, such as that it is “mysterious”. (See the entry on  rationalism vs. empiricism.)  Some philosophers, empiricists and otherwise, have attempted to explain a priori knowledge as arising from some sort of convention or tacit agreement to assent to certain sentences (such as (1)) and use certain rules. Hobbes in his objections to Descartes' Meditations (“Third Objections”, IV, p. 608) proposes a wide-ranging conventionalist view. The later Wittgenstein (on one interpretation) and Carnap are distinguished proponents of “tacit agreement” and conventionalist views (see e.g. Wittgenstein 1978, I.9, I.142; Carnap 1939, §12, and 1963, p. 916, for informal exposition of Carnap's views; see also Coffa 1991, chs. 14 and 17). Strictly speaking, Wittgenstein and Carnap think that logical truths do not express propositions at all, and are just vacuous sentences that for some reason or other we find useful to manipulate; thus it is only in a somewhat diminished sense that we can speak of (a priori) knowledge of them. However, in typical recent exponents of “tacit agreement” and conventionalist views, such as Boghossian (1997), the claim that logical truths do not express propositions is rejected, and it is accepted that the existence of the agreement provides full-blown a priori knowledge of those propositions.