《黄帝内经·素问·平人气象论》
风热为阳邪，脉应浮大，今反沉静；泄利脱血，津血受伤，脉应虚细，今反实大；病在内，脉应有力，乃正气尚盛足以抗邪，今反脉虚；病在外，脉应浮滑，乃邪气仍在于表，今反见脉强坚，以上各种情况都是脉证象反的，因此是难治之病，这就叫做 反四时 。
风热而脉静，泄而脱血脉实，病在中脉虚，病在外脉涩坚者，皆难治，命曰反四时也。
Wind-heat is Yang-evil and the pulse should be large and floating, but now is the opposite-low and calm; profuse sweating and bleeding, the body fluid and blood are injured, the pulse should be weak and fine, but now is the opposite-strong, the disease is internal, and the pulse should be strong, which means that the righteous Qi is still strong enough to resist the evil, but now is the opposite-the pulse is weak; the disease is external, and the pulse should be floating and flowing, which means that the evil Qi is still superficial, but now is the opposite-the pulse is strong and hard. The above are all examples of the pulse and sign being in reverse, so it is a disease that is difficult to treat, and is hence called "the reversal of the four seasons."