Oftentimes, I find the best way of accomplishing something is convincing "myself" to believe something that isn't true. For example, I eat better by convincing my brain that junk food makes me feel sick and I exercise more by convincing my brain that it enjoys pain.

In both of these cases, I'm subverting my brain's natural instinct for self preservation, and I have to ask, is this rational? When my brain wants to eat junk food, it's seeking vital nutrients that historically were hard to come by. When my brain tells me to stop and rest, it's because pain usually means something is in trouble. So, if my brain is being rational by telling me to do these things, am I therefore irrational to suppress or subvert these desires? I'm honestly not entirely sure.

This does lead to another question though: In the preceding paragraph, I allude to a war between "my brain" and "me". Now, what does this mean? How can "I" want something that my brain does not?

These two things, to me, are the crux of rationality. To be rational, one must be able to make the distinction between one's brain (organic, instinct-driven, shaped by evolutionary pressures) and one's self (conceptual, rationality-driven, shaped by societal pressures), recognize which goals are being sought by which, and choose which are worth pursuing.