Bernardo Kastrup

There cannot be. Since knowledge exists only in mind, everything touched by the fingertip of knowledge, no matter how slightly and subtly, is instantaneously ‘brought into’ the domain of mind. Fundamentally, we can never know anything supposedly outside mind. Idealism is, thus, the default metaphysics unless there are substantive reasons to think otherwise.

Materialism requires the following four statements about reality to be true:
1. Your conscious perceptions exist;
2. The conscious perceptions of other living entities, different from your own, also exist;
3. There are things that exist independently of, and outside, conscious perception;
4. Things that exist independently of, and outside, conscious perception generate conscious perception.
Notice that the statements are ordered according to how many new assumptions they require. Indeed, statement 1 is very close to the famous cogito ergo sum, ‘I think, therefore I am.’ If you can be sure of anything at all, it is that your conscious perceptions exist. So statement 1 is the one absolute certainty you can ever have

Statement 3, on the other hand, requires a much more significant leap of faith, since it postulates an entirely new category – namely, things outside conscious perception – for which you can never have any direct evidence. Indeed, everything you can ever know comes into consciousness the moment you know it, so the belief that there are things outside consciousness is an abstraction beyond knowledge