Any justification or knowledge theory in epistemology says that beliefs are justified (known) if they are based on basic beliefs (also called foundational beliefs). Basic beliefs don’t need to be backed up by other beliefs because they make sense on their own. Different beliefs can be supported by fundamental beliefs, which in turn can be supported by other beliefs. Foundationalists say that fundamental beliefs are justified by mental events or states (like experiences) that are not beliefs or are not the kind of thing that can (or needs to be) justified.