Virtue is discerned in the mean [meaning 'average' or 'centre' here]. Accordingly, all evil naturally operates in a deficiency or an excess of virtue. In the case of courage, cowardice is the lack of virtue, and rashness is its excess. What is pure of each of these is seen to lie between these corresponding evils and is virtue. Gregory of Nyssa, Life of Moses, Trans. Abraham J. Malherbe and Everett Ferguson (New York: Harper One, 1978), 121.
Here we begin to see some of how Gregory would describe apatheia. Often this is understood as "passionlessness", but for Gregory it is slightly nuanced. That is, apatheia is the "proper use of passions" (what we might understand better today as our desires); in this case it's not being excessive in what we are passionate about it, or at the opposite extreme completely dispassionate about anything. Rather, the "middle ground" is the ideal for Gregory.
Gregory turns to what we might consider an a obscure verse of Scripture for support here in Matthew 10:16; "I am sending you out like sheep among wolves. Therefore be as shrewd as snakes and as innocent as doves.". This is not an uncommon approach for him (and others), given his penchance for allegory, and its accompanying underlying assumption that if there is not a clear "surface" meaning in the text then the "spiritual" meaning should be pursued more vigorously (hence he heads for books like Song of Songs and Ecclesiastes quickly). For Gregory this verse from Matthew demonstrates perfectly the balance that he is advocating for. It's not shrewdness on its own; as in the case of people who treat others like door mats to achieve their own purposes. Neither is it innocence on its own; as in the case of the ones who allow themselves to become door mats in the name of humility! "Rather, it is the disposition which closely unites these two by the mean that is virtue... The disposition observed in the mean between these two is moderation." (ibid, 120)
This understanding of the Christian life is very applicable today. I hear a lot about "passion". People say they are "passionate" about a whole variety of things; their favourite football team, their family, their work. It's also used in terms of Christian ministry; people are passionate about issues of social justice, evangelism, worship or similar. We can learn the lesson of moderation from Gregory lest our passions control us and not us our passions. These things listed above as examples are all good things, but if any one of them consumes our energies to the detriment of the rest of our responsibilities in life then we run into the danger that was highlighted at the beginning of this post - "evil naturally operates in a deficiency or an excess of virtue." Moderation is the key; that is apatheia - self-control evident in every area of our life, not just negatively speaking over those things that we easily label sin, but also positively with regard to those interests that we are in fact the most passionate about.