If there's one concept that pops into my thoughts a lot, it's opportunity cost. Opportunity cost can be defined as the resources used for something that then cannot be used for something else. For example, if I spend an hour learning Japanese, then I cannot spend that same hour updating my blog. If I spend $10 on a book, I cannot spend it to go to the movies.
This is one of the causes of my undecidedness. If I decide to spend time and money to work on my PhD, I cannot spend that time and money on teaching English in Japan. I tend to look at the estimated total cost, usually just with time and money, when thinking about what I want to do with my life. I want to generalize enough to try and create lots of different things, but I want to specialize enough to make a good enough living to have my own house and the time and money to work on other things.
This is one of the reasons why I've spend a bit of time lately thinking of passive income. By writing ebooks and automating the selling process, I can make money while learning card tricks, or traveling in Japan, or taking a cooking class. I could create a blog dedicated to a single topic, but what topic do I know enough about to spend time and effort on?
Once I know the opportunity cost for something, I can then spend time thinking about the pros and cons. If I do x, spend y dollars and z hours, what do I get out of it? It is personal satisfaction? 5x dollars? Some new bit of knowledge? Do I spend years of my life working toward a PhD just to earn the same salary as before I started, as well as be stuck in a specialty with little time to explore?
My personal question is, "What opportunities do I have where I make a living and still have time to play piano, draw, garden, learn Japanese, take random classes, travel, and, most importantly, start a family?" House husband to a rich woman is the easiest answer. I doubt I'm qualified for that, though.
Sometimes I wish I was one of those who didn't even think about opportunity cost. I could just choose whatever I wanted without thinking of missed opportunities. I guess it's a pain when you want to have more of the pros of life and avoid more of the cons.