Si 思 is a concept in Chinese philosophy that is usually translated as "reflection" or "concentration." It refers to a species of attentive, non-rationalthought that is directed at a specific subject.
↑ Ivanhoe, P.J., & Van Norden, B.W. (eds.) (2001). Readings in Classical Chinese Philosophy, 2nd Ed. Hackett Publishing Co.: Indianapolis, p. 392