ABSTRACT
It will be remembered that we regarded perception, in Chapter 5, as a species of ‘sensitivity’. Sensitivity to a given feature of the environment we defined as consisting in some characteristic reaction which is exhibited whenever that feature is present, but not otherwise; this property is possessed more perfectly, in given directions, by scientific instruments than by living bodies, though scientific instruments are more selective as to the stimuli to which they will respond. We decided that what, from the standpoint of an external observer, distinguishes perception from other forms of sensitivity is the law of association or conditioned reflexes. But we also found that this purely external treatment of perception presupposes our knowledge of the physical world as a going concern. We have now to investigate this presupposition, and to consider how we come to know about physics, and how much we really do know.
