 those qualities are in the mind only as they are perceived by it, that is, not by way of mode or attribute, but only by way of idea; and it no more follows, that the soul or mind is extended because extension exists in it alone, than it does that it is red or blue, because these colours are on all hands acknowledged to exist in it. (Principles, pt 1, art. 49; see also Three Dialogues, Third Dialogue [2008: 229–230])