We can distinguish the reasons of why one thinks that something is such and such, and the reasons why the thing is such and such.
In the post “Does Principle Of Sufficient Reason Imply Idealism?“, I said that idealism is optimistic philosophy in that that it considers that it is in principle possible to understand the reasons why the things are such and such. And in case where we actually understood why the things are such and such, the reasons why we believe that the thing is such and such, says the idealist, are same with the reasons of why the thing is such and such. Consider some physical law P1. The pragmatic reasons for believing that the world is such and such, i.e. that P1 holds in our world are based on empirical research. The idealist however, in the childish optimistic spirit because of which she got into philosophy in first place, asks “Why?”. She says – “If P1 holds in our world, there is surely a reason why it holds, and it is no pragmatic reason.”. (This is empirical proof that the idealist is made of same stuff as those 3 year old kids, which ask “Why?” again and again to every explanation they get.)
What kind of kid the empiricist was when she was 3? Some people, opposing science to philosophy, say that sciences answer “how?” questions, and philosophy is about “why?” questions. But doesn’t science explain to us why there are such things as lightnings, doesn’t it explain to us why people get sick, doesn’t it explain why we have winter (though not much of it this year), and why we have summer, or why is there so much different species in the world?
So, it can’t be a difference between why and how. In fact while the empiricist has pragmatic reasons for her belief that P1 holds in the world, the physical law is not what she is trying to explain. She is using P1, in order to explain phenomena in the world. But with this on mind, as much as empiricist is using her pragmatic belief that P1 holds in the world to explain what is happening in the world, we can say that the empiricist is idealist!
Namely if empiricist wants to claim that her explanations of the world might hold, she will have to admit that what she believes are reasons for the phenomenon being such and such, might actually be the reasons why the phenomenon is such and such. And it is clear (or at least it seems clear to me) that this assumes the possibility of identity of world and thought, of the reasons as thought and the actual reasons. If this was not the case, then empiricist’s explanations wouldn’t be about the world.