Dogen on the koan Tozan's Heat and Cold

Great Master Tōzan Ryōkai was once asked by a monk, "When cold or heat come our way, how are we to avoid them?"
The Master replied, "Why don’t you proceed to the place where there is no heat or cold?"
The monk then asked, "What is that place where there is no heat or cold?"
The Master answered, "When it is cold, my acharya, give yourself up to the cold; when it is hot, my acharya, give yourself up to the heat."

Ever so many people in the past have given much thought to this dialogue, and many people in the present need to make every effort to train with it. The Buddhas and Ancestors have invariably come to explore it, and those who come to explore it are Buddhas and Ancestors. Many of the Indian and Chinese Buddhas and Ancestors in both the past and present have treated this story as an actual incidence of one’s Original Nature manifesting. In this dialogue, Original Nature manifests as the spiritual question of Buddhas and Ancestors.
Shobogenzo Shunju, Rev. Hubert Nearman, p.746