Encyclopedia of Philosophy: THE TRANSCENDENTAL DIALECTIC: THE ANTINOMIES OF PURE REASON - KANT

We turn next to a priori cosmology. Here Kant presents us with a set of antinomies. An antinomy is a pair of contrasting arguments which lead to contradictory conclusions (a thesis and an antithesis). Kant constructed a set of these to show that any attempt by reason to form ‘cosmical concepts', that is to say, notions of the world as a whole, was bound to lead to irresoluble contradiction.  The first antinomy has as thesis ‘The world has a beginning in time, and is also limited as regards space' and as antithesis ‘The world has no beginning, and no limits in space; it is infinite as regards both time and space'.