# Day 4 I got depressed when I resisted playing video games, and I went on a day-long quest of observing myself and diagnosing what caused it. To bring in here some of what I learned from statistics, this is in fact an interesting causal inference problem: I am only aware of the symptoms and I have to deduce the causes. Statistics makes one wary of correlations – to oversimplify, things happening next to each other aren't necessarily causing one another. In my case, I could have attributed my lethargy to any combination of lack of caffeine, lack of exercise, yesterday's heat stroke from overexercising, and a myriad other potential causes. I don't think I'll be knowing this one for sure, but I hypothesize it was largely because of video games. I'll have to be aware of my changes and react accordingly. Caffeine helped elevate my mood in the morning but I was feeling down after lunch. Usually I resort to playing video games when I get bored – bad mood seems to be a trigger of this habit. By now I have mostly succeeded in substituting exercising for my old gaming habits. Onwards.