The human mind is wired in a deterministic way. A proposition is either necessarily true or it is false.
For example, you either like a person or you don’t. There isn’t an in-between where you would say that there is a 67% probability you like someone. The same principle applies to investing, we either think the market or a stock are going to go up or down. What very few can do is to think in a probabilistic way. But mastering a probabilistic way of thinking would do wonders for your portfolio.