 The factors that make us believe in external existence of objects of perception are said to suffice for assent to their independence. Belief that objects are perceivable by more than one human being tends to produce belief that their existence is independent of any particular human being. A second source of belief in independence is that we infer that while objects exist unperceived they undergo change; a fire viewed at one time and again an hour later may have died down in the meantime. The most important source is that we perceive objects whose sensory appearance is very like that of ourselves. We suppose the appearances are caused by things like ourselves and infer that they are like us in having sensations and thoughts. Since their thoughts and sensations are not perceived by us, this leads to the belief that their existence is independent of ours. But although Shepherd clearly maintains that these arguments affect our credence, none of them is definitive proof that objects are independent of an individual mind.