The presence of the concept of judgement in many parts of Descartes' philosophy, methodology, metaphysics and morals has been widely remarked upon. But the reason why judgement is all-pervasive in Descartes' work has not been elucidated. This is the issue at stake in the present study of the nature and the function of the concept of judgement in the Regulae ad directionem ingenii, a text Descartes is likely to have been working on from 1618 to 1628 and which was not published during his lifetime. The Regulae, which belong to the earliest strata of Descartes' work, constitute the starting point not only of Descartes' scientific thought but also of his philosophy in general.