A modern conception, there is no mentioning of a "heavy cavalry" in the Greek military manuals. Unfortunately, even today, we don't have a concrete notion of what "heavy cavalry" should be. According to one school of thought, it is any cavalry capable of shock action against the enemy line, according to another, it should just be heavily armored. According to the Greeks, we have to define it as any cavalry that was not considered "light", that is, which was not purely skirmishing. Another aspect of the cavalry of the ancient era we have to keep in mind is the unwillingness to use even the best trained and heaviest of cavalries against any dense mass of able infantry. This is evident in many ancient descriptions of battles. According to Arrian, when Alexander faced the Indian tribe of the Malli, he did not dare assault them with his, by now, veteran Heteroi or Thessalians, but he followed the customary cavalry tactics of attacks and retreats (perispasmoi).