A conceptual fallacy is an error in one's approach to the world or in one's thought process about the world.
Cart before the horse
People commit this fallacy when they first assume that a concept is true, and then search for proofs of the concept. In such a situation, people tend to distort all the data available to them in order to fit the assumed concept. In other words, they are not seeing the world as it is, but as they want it to be. This fallacy is related to the begging the question logical fallacy.
The order, of first assuming a conclusion, and then attempting to prove that conclusion, is backwards. The correct order would be to first look at the data and then come to whatever conclusion that the data leads. Alternatively, if the concept is truly basic, one can assume it, in which case it requires no proof. In principle, one can appear to avoid the whole issue by assuming all of his positions. But the more assumptions, the greater the likelihood that they are false.