Every dictionary contains a finite number of terms. Each term defines itself by using other terms (also in the dictionary). So in a very real sense, all definitions are circular. Meaning, which is associated by the reader with certain words, comes from thought and experience. I know what "red" is because I have seen red things. I know what delusion is because I have experienced it. I know what truth is because I directly apprehend the concept.
Every argument, or proof, is finite. It therefore necessarily begins with unproven or assumed premises. These premises may be random and arbitrary, as in some"formal systems", or they may be provisional or working premises; or they may be self-evident and directly apprehended, and therefore not in need of proof; or they might seem obvious but upon scrutiny can be seen to depend upon auxiliary assumptions that were not even explicit.
If you expand the argument to prove the originally unproven premises, the new expanded proof will itself, being finite, begin with unproven premises. So every proof or argument depends on premises that cannot be proven by proofs or arguments, but only through direct apprehension of certain basic truths.
Corroboration by other parties is worthless unless you can establish that those other parties are: (a) real rather than illusions, and (b) neither lying individually nor in collusion, nor mistaken, nor mutually influenced by mistakes or lies.
Historical evidence is worthless unless you can establish that what purports to be history isn't fiction or distortion.
Perceptual experience is what it is, but may not be what it is interpreted as. Someone dreaming or hooked up to a computer simulation has definite perceptions but what they are evidence of is another question.