Complex sentence: Chrysippus preferred to regard it as an alteration or change in the soul; that is, the soul receives a modification from every external object that acts upon it, just as the air receives countless strokes when many people are speaking at once.
Split into simpler sentences: Chrysippus preferred to regard it as an alteration or change in the soul. The soul receives a modification from every external object which acts upon it, just as the air receives countless strokes when many people are speaking at once.