Note 51 — Chapter III
Reading edition
I may here observe that the earliest age in the infant at which I have observed such appreciation of causality to occur is during the sixth month. With my own children at that age I noticed that if I made a knocking sound with my concealed foot, they would look round and round the room with an obvious desire to ascertain the cause that was producing the sound. Compare, also, Mental Evolution in Animals, pp. 156-158, on emotions aroused in brutes by sense of the mysterious—i.e. the unexplained.