Hermann von Helmholtz

I then endeavoured to show that it is more especially in the thorough conformity with law which natural phenomena and natural products exhibit, and in the comparative ease with which laws can be stated, that this difference exists.

Heat can also be produced by the impact of imperfectly elastic bodies as well as by friction. This is the case, for instance, when we produce fire by striking flint against steel, or when an iron bar is worked for some time by powerful blows of the hammer.

Self-indulgence is something to watch, but anything that helps you understand situations that are difficult to understand is good. If you're having some sort of emotional trauma, you need to find a person to talk to about it who says, 'This is quite normal; it's fine.'