In other words, positive thinking is very helpful, possibly even a necessary precondition for some outcomes (like becoming president - ain't gonna happen unless

In other words, positive thinking is very helpful, possibly even a necessary precondition for some outcomes (like becoming president - ain't gonna happen unless you really do want it), but it is neither a predictor nor a principal factor. On the other hand, it is arguable that negative thinking can be a fairly strong predictor of failure. That is perhaps the most useful take-away from the otherwise relatively useless field of positive thinking: if you keep thinking you're going to fail, keep visualising and expecting failure, chances are very good that you will indeed fail, even if you had all the other necessary factors for success.