Emotion

Humans are amphibians – half spirit and half animal.

As spirits they belong to the eternal world, but as animals they inhabit time.

This means that while their spirit can be directed to an eternal object, their bodies, passions, and imaginations are in continual change, for to be in time means to change.

Their nearest approach to constancy, therefore, is undulation – the repeated return to a level from which they repeatedly fall back, a series of troughs & peaks.

If you watch carefully, you will see this undulation in every department of his life – his interest in his work, his affection for his friends, his physical appetites, all go up & down.

As long as he lives on earth periods of emotional and bodily richness and liveliness will alternate with periods of numbness & poverty.

Fun is closely related to Joy – a sort of emotional froth arising from the play instinct; it promotes charity, courage, contentment, and many other evils.

THE SCREWTAPE LETTERS by C.S.LEWIS