He’s talking about inversion, the idea that problems are often best solved when they are reversed. That it’s often easier to think about what you don’t want than what you do. In his own words:
9. Don’t compete with others. Make them compete with you.
…to make sure of our ground and be quite certain that we really do like a thing before we say we do. When you cannot decide whether you like a thing or not, nothing is easier than to say so and to hang it up among the uncertainties.”