  In reply, Descartes claims that he has already supplied such a method (7:379). What could he have in mind? It cannot be the simple belief  that one has attained clarity and distinctness, for Descartes himself acknowledges that individuals can be wrong in that belief (7:35, 361). Nonetheless, he does offer a criterion. We have a clear and distinct perception of something if, when we consider it, we cannot doubt it (7:145). That is, in the face of genuine clear and distinct perception, our affirmation of it is so firm that it cannot be shaken, even by a concerted effort to call the things thus affirmed into doubt.