SENSATIONS are the primary events of the mental life. They are so called because they arise through the senses. We use the word in its usual sense, as meaning the great body of psychological phenomena, both affective and presentative, which result within the mind immediately from impressions upon the senses. The experiences of moisture and resistance which follow from contact with a piece of iron, and the pain felt in case it is hot, are equally sensations.
Distinction between Sensation and Impression. Sensation being thus defined, it must be carefully distinguished from the physical phenomenon which precedes or accompanies it. The impression is the modification of the organ, especially of the nerves and nervous centers, which arises from an external stimulus; as the vibration of ether or air. The nature of the different sense impressions is not well understood; but in each case they are some form of movement. They have all the characteristics of physical phenomena: they can be located, measured, apprehended by the senses. Sensations, on the other hand, cannot be compared with movement of any kind. The difference between____________________