We can see very well why the translator rendered 'plane' by 'raisonnablement'; in attempting to carry out this rationally necessary doubt, I expose myself to losing the qualification of 'raisonnable' that I brought into play from the very beginning of the meditations (and in at least three forms: having a mind that is sufficiently mature, having a mind that is free of cares and passions, and being sure of a peaceful retreat).