Page:Russell - An outline of philosophy.pdf/36

24 Unless all four stages exist, there is nothing that can be called "knowledge". And even when they are all present, various further conditions must be satisfied if there is to be "knowledge". But these observations are premature, and we must return to the analysis of our third and fourth stages.

The third stage is of two sorts, according as we are concerned with a reflex or with a "learned reaction", as Dr. Watson calls it. In the case of a reflex, if it is complete at birth, a new-born infant or animal has a brain so constituted that, without the need of any previous experience, there is a connection between a certain process in the afferent nerves and a certain other process in the efferent nerves. A good example of a reflex is sneezing. A certain kind of tickling in the nose produces a fairly violent movement having a very definite character, and this connection exists already in the youngest infants. Learned reactions, on the other hand, are such as only occur because of the effect of previous occurrences in the brain. One might illustrate by an analogy which, however, would be misleading if pressed. Imagine a desert in which no rain has ever fallen, and suppose that at last a thunderstorm occurs in it; then the course taken by the water will correspond to a reflex. But if rain continues to fall frequently, it will form watercourses and river valleys; when this has occurred, the water runs away along pre-formed channels, which are attributable to the past "experience" of the region. This corresponds to "learned reactions". One of the most notable examples of learned reactions is speech: we speak because we have learned a certain language, not because our brain had originally any tendency to react in just that way. Perhaps all knowledge, certainly nearly all, is dependent upon learned reactions, i.e. upon connections in the brain which are not part of man's congenital equipment but are the result of events which have happened to him.

To distinguish between learned and unlearned responses is not always an easy task. It cannot be assumed that responses which are absent during the first weeks of life are all learned. To take the most obvious instance: sexual