As I've posted before, I do strongly believe that there are human elements to the game that can't be quantified. I believe there is such a thing as "grit". IMO, a perfect example is Pedroia. How many critics have doubted him througout his entire career because of his size? I believe that part of what makes him as successful as he is is his grit, or confidence, or detemination, whatever you want to call it.
Do I have stats to prove that? No. OTOH, has anyone shown me any stats to disprove that? No. So this is my opinion based on just my belief that "grit" does exist. If someone told me that grit has nothing to do with Pedroia's success, I would disagree, but I would not be able to say that they are wrong on that topic.
There is a difference when you talk about momentum. Momentum does exist. A team that has won 8 games in a row has momentum. However, the numbers have shown that there is no predictive value to momentum. In other words, saying a team will carry momentum into the next series is a false statement. A team that was on a hot streak is just as likely to lose the next series as it is to win, relative to its overall talent level. This fact has been quantified.