 Frege’s main criticism of psychological logic is that it conflates ‘true’ and ‘being-taken-to-be-true’. To begin with, Frege denies that prescriptions based on psychological laws can qualify as proper logical laws. Such prescriptions can be no more than demands to conform to current thinking habits. But they cannot be yardsticks by which these thinking habits can be evaluated as to their truth. Moreover, Frege points out that the descriptive psychological laws which for the psychological logician provide the basis for (psycho-)logical prescriptions are laws of ‘taking-to-be-true’: they state the conditions under which humans accept the truth of judgments or the validity of inferences; but they do not determine the conditions under which judgments are true and inferences valid. Here Frege is particularly scathing of the psychological logician Benno Erdmann who identifies truth with general consensus. For Frege this move makes truth dependent upon what-is-taken-to-be-true. And it fails to give proper heed to the insight that truth is independent of people’s agreement. It follows that the laws of logic are not psychological laws: ‘If being true is thus independent of being acknowledged by somebody or other, then the laws of truth are not psychological laws: they are boundary stones set in an eternal foundation, which our thought can overflow but never displace’ (1893, XVI). Frege’s attack on Erdmann does not end here. Erdmann is also taken to task for his suggestion that logical laws might have mere ‘hypothetical necessity’, that is, that they are relative to the human species. Frege maintains that if we were to encounter creatures who deny the laws of logic, we would take them to be insane; and he analyses Erdmann’s proposal as reducing, yet again, truth to what-is-taken-to-be-true (1893, XVI–XVII). At the same time, Frege accepts that the most fundamental logical laws cannot be justified. Logical justification comes to an end when we reach these laws. However, to argue that our nature or constitution forces us to abide by the laws of logic is no longer a logical justification; it is to incorrectly shift from logical to psychological or biological considerations (1893, XVI–XVII).