Kant talked of “the thing in itself”. Like Plato he suggested that every object had a real essence but its essence was transcendent; it was beyond human apprehension. When he talked of the thing in itself he meant the real thing as opposed to the thing as experienced by the senses. By this account things had their true existence outside of experience. This is plausible enough. If no sentient beings had ever inhabited the world would it still exist? Presumably yes. In other words it would have an existence independent of any experience of it.

In Kant's transcendent world, what he called the noumenon, everything exists in its purest essence beyond experience. It was an idea criticised by Schopenhauer who said that whatever the noumenon was it would not be differentiated - i.e. it would not be made up of separated entities. Differentiation was a facet of the phenomenon not the noumenon. In the noumenous world, there were no individual identities. The noumenon was likely to be some kind of non-differentiated totality.

Wittgenstein trumped these notions. For him, by definition these things are unknowable. They are best left as such. Does the tree make a sound when it falls and nobody hears it? This is unknowable. Would the universe have existed in the absence of sentience? Also unknowable. It seems plausible to say yes it would have but that would be a matter of faith.