The problem I shall discuss is What reason have we for believing that there are physical objects? My purpose is not either to raise or to dispel doubts as to the existence of physical objects; this doubt constitutes a medical rather than a philosophical problem. The point of asking the question is that, while there can be no reasonable difference of opinion as to whether there are physical objects, there can be and is reasonable difference of opinion as to how the notion of a physical object is to be analysed; and if we are clear as to what grounds there are for believing in physical objects, we shall also be clearer as to what sort of physical objects we have grounds for believing in. Also, it is worth while to inquire which other beliefs are logically connected with, and which are logically independent of, the belief in physical objects.