His major work is the Muqaddimah or An Outline of History, from which the following translation is taken. They are found eager to dance whenever they hear a melody. They are everywhere described as stupid. The real reason for these opinions is that, as has been shown by philosophers in the proper place, joy and gladness are due to expansion and diffusion of the animal spirit.
Sadness is due to the opposite, namely, contraction and concentration of the animal spirit.
It has been shown that heat expands and rarefies air and vapours and increases their quantity. A drunken person experiences inexpressible joy and gladness, because the vapour of the spirit in his heart is pervaded by natural heat, which the power of the wine generates in his spirit. The spirit, as a result, expands, and there is joy. Likewise, when those who enjoy a hot bath inhale the air of the bath, so that the heat of the air enters their spirits and makes them hot, they are found to experience joy. It often happens that they start singing, as singing has its origin in gladness.