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I started keeping a diary about a month ago. The two initial reasons I had for adopting this habit were that, first of all, I thought that I would enjoy writing, and second of all, I wanted to have something relaxing to do for half an hour before my bedtime every evening, because I often have trouble getting to sleep at night.
I have found that I generally end up writing about my day-to-day social interactions in my journals. One really nice benefit of keeping a journal that I hadn't expected to reap was that writing has helped me weakly precommit to performing certain actions that help me improve at being sociable. For example, a few weeks back, there were a couple nights where I wrote about how I felt bad about how a new transfer student to my school didn't seem to know anyone in the class which we had together. A couple days after writing about this, I ended up asking him to hang out with me, which was something that I normally would have been too shy to do.
Another thing that I learned is that writing about your problems can help you digest them in ways which are helpful to you. On a meta- level, I think that writing about my social interactions with others has helped me realize that I want to spend more time with my friends, at the expense of spending less time reading through e.g. posts on Reddit. Looking back on things, it is painfully obvious to me that spending time with my friends is much better than spending time on random internet sites, though I hadn't explicitly realized that I had been failing to spend time with my friends until I ended up writing about the fact that this was the case.
Actually, before I had even started journaling, I had known that thinking about problems by writing about them or making diagrams was, in general, a helpful thing to do-- after all, plenty of people benefit from drawing pictures when stuck on, say, math problems. However, it wasn't previously obvious to me that problems other than math and science problems could be analyzed by writing about them or drawing diagrams that represented the problem. Basically, I found a way (which was previously unknown to me) to identify and solve problems in my life.
No such thing.
For any given problem, once a possible solution is reached, do you expect to be able to check that solution against reality with further observations? If so, you have constructed a theory with experimental implications, and are doing Science. If not, you have derived the truth, falsehood, or invalidity of a particular statement from a core set of axioms, and are doing Math.