 Whether or not a particular theoretical (or practical) choice is susceptible to an internal explanation depends, in part, on the actor’s problem. Consider, for example, Descartes’ theory of the vortices, namely that the planets are whirled round the sun by a fluid medium which itself contains little whirlpools in which the individual planets are swimming. Descartes’ theory of the vortices, is fairly rational if we take it as an attempt (in the light of what was then known) to explain the motion of the heavenly bodies in a way that is consistent with Copernican astronomy. But it is a lot more rational if we take to be an attempt to explain the motion of the heavenly bodies in a way that is consistent with Copernican astronomy without formally contradicting the Church’s teaching that the earth does not move. (The earth goes round the sun but it does not move with respect to the fluid medium that whirls it round the sun, and, for Descartes, motion is defined as motion with respect to the contiguous matter.) So do we read Descartes’ theory as a fairly rational attempt to solve one problem which is distorted by an external factor or as a very rational attempt solve a related but more complex problem? Well the answer may not be clear, but if we want to understand Descartes intellectual development we need to know that it was an important constraint on his theorizing that his views should be formally consistent with the doctrines of the Church.