Whether animal projects are ethically fungible, and if they are, in which cases, is a matter of both public debate as well as academic discussion in moral-philosophy. Also, the approving authorities are instructed to evaluate animal projects in light of their,ethical fungibility, as a prerequisite for their permission. The question of ethical fungibility therefore is situated in a hybrid position between ethics and law. Consequently, it often remains unclear as to how far social and ethical responsibility as well as authority in the evaluative action could or should reach. Applying Kant's distinction between a public and a private use of reason can help to disentangle the limits of these two spheres of responsibility. The private use of reason corresponds to the use of reason that is determined by legal frameworks, as required in evaluative action. Public use of reason, in contrast, is not limited in neither formal nor legal regards. The question of ethical fungibility requires both forms of use of reason, without confusing nor reducing them with each other. The consequences of Kant's distinction for the evaluation of ethical fungibility will be explained with a special focus on the evaluation of the proposed project's benefit.The example of the evaluation of research on lifestyle-related diseases will show, how the evaluation of ethical fungibility calls for a careful distinction between public debate of normative judgements and official acquittal.The high requirement on ethical justification of animal projects is thereby not narrowed, but rather raised by a clarification of cognisance and conditions.