The general military doctrine of Thesos was that it was mass infantry upon mass infantry that decided the fate of any battle, while cavalry only played second fiddle, working to harass the opponent, protect the infantry's flanks, and chase down the fleeing enemy once the fight was over, thus inflicting massive casualties to these out of formation units and then rendering them unable to fight any subsequent battles.
But this line of thought was in much contrast to Alexander's own doctrine for he really believed in the opposite.
He believed that an infantry's only job was to pin down the enemy, while the heavy cavalry did all the heavy work, swinging around and charging, preferably at the enemy's exposed flanks, thus destroying them.
Now who was right in this you ask?
Well, it really depended on how you looked at it.
For both men's mindsets were a product of their time.