Wittgenstein's Tractatus contains an insubstantial form of transcendental idealism. It is insubstantial because it rejects the substantial a priori. Yet despite this, the Tractatus still contains two fundamental transcendental idealist insights, (a) the identity of form between thought and reality, and (b) the transcendental unity of apperception. I argue for (a) by connecting general themes in the Tractatus and in Kant, and for (b) by giving a detailed interpretation of Tractatus 5.6ff., where Wittgenstein talks about solipsism and the metaphysical subject. Tractarian solipsism, on this interpretation, is a special, insubstantial form of transcendental idealism.