There is nothing that in principle cannot be explained.
So for example, in seeking a general theory of psychology all
that is human is to be embraced and so explained by the theory.
When beginning this process it very quickly becomes apparent that
we need to first establish what we mean by 'explanation'. But
ignoring these technical difficulties for the moment, the proposition
means that we approach the universe with an open mind making a
minimum of pre-judgements of any situation and where we do make
pre-judgements, making it clear to ourselves what they are.
Specifically, in the example of a general theory of psychology
if there is a soul as something carrying the spark or core of
us, then this will become part of the theory. If, however we can
achieve explanation without the concept of soul then it is excluded.
If we design an adequate process, then we do not have to make
the decision in advance, merely to apply the process of theory
creation and follow the design of the theory as it emerges.
In this way the philosophy also defines ontology that is a theory
of what exists and what does not exist in the universe. At least
ontology related to what we currently know and understand.