David Hume doubts the possibility and validity of inferring from "is" to "ought to". While the academic circle gives a theoretical answer of the logic of "ought to", philosophical counselors provide the thought treatment mode of "a practical syllogism" starting from "is" when facing the perplexity of "ought to". We should combine the forward approach of inferring from "is" to "ought to" and the inverse one of tracing from "ought to" back to "is", and integrate theoretical research and practical application so as to get a deeper theoretical understanding of Hume's question on the one hand, and apply academic wisdom in daily life and the care of the mind of common people on the other.