Say you were to come upon a mathematical equation whose variables and sheer complexity were beyond your knowledge at that point in time. As an example, a simple algebraic problem may seem to be a completely different language to young children who have hardly mastered their multiplication tables. Note that I said an "equation" and not a mathematical problem.
This means that you see two sides, neither of which are wholly understood by you, that have been determined to be equated.
Despite the fact that you don't understand it, you continue to build your knowledge based only on what you have already learned, and one day, the equation finally makes sense.
In a realist or solipsistic view, reality is either generated by your own mind or illusory altogether. How, then, can you 'think up' an equation or be presented one illusorily, that makes no sense to one's self, yet build on rules that you had previously known (or, rather, believed to have known) to determine that the original equation is true?