According to Proust, ideas which have arisen without pain lack an important source of motivation. Mental activities can thus be divided into two categories. On the one hand, there are “painless thoughts”, sparked by no particular discomfort, e.g. having a disinterested wish to find out how sleep works. On the other hand, there are “painful thoughts”, arising out of a distressing inability to do something, e.g. not being able to sleep due to insomnia.