"Schematically, the argument is as follows. Berkeley says: “Sensible objects must be sensible. A is a sensible object. Therefore A must be sensible.” But if “must” indicates logical necessity, the argument is only valid if A must be a sensible object. The argument does not prove that, from the properties of A other than its being sensible, it can be deduced that A is sensible. It does not prove, for example, that colours intrinsically indistinguishable from those that we see may not exist unseen. We may believe on physiological grounds that this does not occur, but such grounds are empirical; so far as logic is concerned, there is no reason why there should not be colours where there is no eye or brain."