Arguments from illusion take as premises either the existence, or the possibility, of illusions, and draw as conclusions either the possibility of total illusion, or scepticism concerning claims to knowledge. Thus the senses sometimes deceive us, and may do so on any occasion; hence, it is argued, perhaps they always deceive us, or at any rate we should never trust them implicitly. In some forms the arguments are undoubtedly invalid: thus, even if it is true that the senses may deceive us on any occasion, it does not follow that they may deceive us on every occasion. Any coin may be counterfeit, but it cannot follow that all coins together may be counterfeit, since forgeries have to be parasitic upon the real thing. Illusions have always been a starting-point for epistemology, and motivate the foundationalist desire for absolutely certain starting-points, free from their possible contamination. See also method of doubt.