I always thought of it in the sense of a Taylor expansion. First-order thinking suggests that every system reacts linearly according to whatever the dominant f

I always thought of it in the sense of a Taylor expansion. First-order thinking suggests that every system reacts linearly according to whatever the dominant factor is (the way you would basically expect). For example, increase prices and your sales will decrease. Second order thinking suggests including the next-more-complicated effect in your model; effects that are negligible for small perturbations have to be taken into account. Increase prices and your product gets perceived as more exclusive and higher quality, and despite the valid decrease that the first-order effect predicts, sales go up.