Commentary Par IV 25-27

Both of the protagonist's questions reflect a dangerous uncertainty on his part about the nature of the free will, which will be the subject of Beatrice's urgent lecture at the beginning of the next canto.  If he believes that compromise in making vows is possible or that our souls' choices are controlled by the stars that govern our natures, he is in heresy.  The second doubt, since it would destroy the notion of free will utterly (and not just partially), is the more dangerous, which is why Beatrice chooses to address it first.   As Trucchi, discussing this tercet (DDP Trucchi.Par.IV.25-27), points out, Plato's notion that souls return to their formative stars (embraced by some early Christians), was finally condemned at the Council of Constantinople in the year 540.