I recently was re-reading excerpts from the best book on preaching. In it, Keller quotes the philosopher Charles Taylor, in A Secular Age, in which he says that “no one has a right to interfere with me for my own good, but only to prevent harm to others.” Taylor was summarizing what might be called the harm principle. It’s the very common notion that we don’t need to judge the value of an idea on the basis of ethics or truth, but only the harm it may or may not cause to others. What one does in the privacy of one’s home is no concern of yours if they’re not hurting anyone.

“You believe what you want to believe and I’ll believe what I want to believe. As long as neither one of us is harming anyone, both of our beliefs can be held as equally valid.” Aside from the fact that not all ideas are equally valid (think, for instance, of Jnco jeans or Taco Bell’s breakfast menu), the truth is that many ideas are actually quite harmful. There is a real level of self-harm in bad ideas. This understanding is baked deep into the Christian tradition.

Gregory of Nyssa, one of the three great Cappadocian fathers, wrote a handbook for catechists. Catechists are skilled in catechesis, the question-and-answer instruction of the faith (including the Trinity, creation, sin, salvation, sacraments, etc.) In Gregory’s Catechetical Discourse, he begins by telling his catechists that, when dealing with different ideologies (whether of a Jewish or Greek persuasion), they are doing “battle with their conjectures.”

In other words, everyone brings presuppositions to the faith (often unconsciously) and the catechist is to unearth them and do battle. Note that Gregory calls for battle with ideas, and not with the person holding the ideas. That’s a crucial distinction often lacking in the current civil discourse. You don’t berate the patient for contracting a disease through no fault of their own.

For the catechist, the conjectures to battle with the harm principle is that 1) we all know what constitutes “harm” and 2) we can define harm without going back to ethics or truth. If we’re not sure what makes something right or wrong, we can’t be sure of what is harmful or harmless. But there’s a third presupposition. To say that what one does in the privacy of one’s home makes no difference to others so long as it’s not hurting anyone fails to acknowledge the reality of self-harm.

It’s on this issue that the Cappadocian makes a crucial observation. As you clear the objections and errors and heresies out of the way, you treat different ideas as distinct because “the manner of healing must be suited to the form of the illness.” An error, Gregory says, at the fundamental levels of God’s nature, or the self, or the work of Christ, are simply too dangerous to be anything other than an illness.

Theologically speaking, ideas that deviate from the revelation of God are like a sickness. Sickness needs to be healed. Debate and argument at this level and in this context are more akin to therapy and medicine than to warfare and dominance.

This completely flips the concept of harm as a guiding principle where one is trying to live out the implication of one’s ideas. If a bad idea is an illness, belief in that idea becomes poison. “You drink your rat poison and I’ll drink my rat poison and no one will get hurt.” Heresy  is not a bad idea to be tolerated, but a sickness of which one should be cured. That is the great value of catechesis. It is deep medicine for a deadly, but curable illness.