Everything you know about the external world arrives through your own experiences, so there is no way to compare your mental picture of reality against reality itself from a vantage point outside your mind. This leaves open the radical possibility that nothing exists beyond your experiences, a hypothesis you cannot disprove from the inside.
The taste of chocolate or the sensation of pain has an inner felt quality that seems to escape any complete account given in terms of brain states and physical processes. Even a full physical description of what happens in the brain would leave out what the experience is like from the inside.
If every action is the inevitable result of prior causes stretching back before your birth, then in no case could you have done otherwise, which seems to undermine blame and praise. Yet if actions are instead random rather than caused, that does not restore responsibility either, leaving free will squeezed from both sides.