"As early as 1901, the German psychologist Karl Groos discovered that infants express extraordinary happiness when they first figure out they can cause predictable effects in the world, pretty much regardless of what that effect is or whether it could be construed as having any benefit to them. Let’s say they discover that they can move a pencil by randomly moving their arms. Then they realize they can achieve the same effect by moving in the same pattern again. Expressions of utter joy ensue. Groos coined the phrase “the pleasure at being the cause,” suggesting that it is the basis for play, which he saw as the exercise of powers simply for the sake of exercising them."
— David Graeber: Bullshit Jobs
This resonated with me. Often if I find myself feeling down, a fairly reliable way to perk myself up again is to do something which is solely an exercise in my personal volition (as opposed to doing something I was told or expected to do, or even invited to do, or even just something that I had previously decided/planned to do).
