This work intends to establish the foundation for furthering the epistemic analysis of legal controversies regarding tax law and particularly the phenom-enon of tax avoidance. Starting from the diagnosis that there is little interest on the part of the doctrine in dealing with the particularities of interpreting facts on this subject, the intent is to relate how facts of tax relevance should be construed from linguistic uses rather than legal uses, adopting the devel-opments of rationalism in evidence studies. This is the only way to build a rationally controllable conceptual apparatus capable of guiding the behavior of the targets of laws.