Galileo is here expressing what has become called the distinction between the primary and the secondary qualities of material things. The secondary qualities are the immediate objects of the senses: colours, tastes, sounds, odors, feels. According to Galileo they ‘hold their residence’ only in the sensitive (i.e. perceiving) animal. Moreover, according to Descartes, there is no reason for supposing them to ‘resemble’ whatever in nature causes them—the arrival of photons at the eye, in the case of colour, for example.
