There are various way to answer the question of “what exists”. There is:

1. materialism (that reality is fundamentally matter);
2. idealism (that reality is fundamentally mind or immaterial);
3. dualism (that reality is both mind and matter);
4. monism (only one kind of thing exists);
5. pluralism (multiple kinds of substance exist);
6. empiricism (only things that can be observed exist or measured exist);
7. rationalism (reason determines what exists);
8. phenomenalism (only perceptions and experience exist);
9. existentialism (existence precedes essence);
10. nihilism (nothing has inherent existence).

All of these ultimately have to explain how it relates to the physical reality in which all actions (including philosophising) occur.

Since the physical world is common to all answers about what exists, we should answer all questions in ration to the physical world.