However, if we see our concepts of reality as established by convention, then this leads us to an "infinite regress". For example, if we agree to call a nose a nose, then we establish a convention. But, to have a concept of a nose, I must first have other concepts (MY nose, FACE, MINE and YOURS, etc.). Therefore, we would have to agree what we mean by THOSE concepts - to a point where we face a paradox: in order to communicate, we need to establish what we mean by certain concepts, but to do that, we need to have concepts that we agree on the meaning of!
So, conventionism works up to a point (after basic concepts are established), but it cannot act as account of fundamental concepts. The interesting question is therefore how these fundamental concepts originate in the first place...