Abstract: The law of the substitution of co-referential expressions is a basic principle of classic logic；however，it is faced with many problems．Frege holds  that  intensional context  invalidates  this principle；Russell seeks a solution  to  its  identity paradox with his description theory；Wittgenstein totally negates the validity of the principle，regarding it as pseudo．This paper argues for the validity of the principle． The discussion shows  that  the arguments of Frege，Russell and Wittgenstein can be  falsified．As a matter of  fact，intensional context  is external to a proposition．When it is introduced into the proposition it becomes an integral part of it，and there is no change of truth value when co-referential expressions are substituted so long as the proposition is one and the same，i. e．the law of identity is obeyed．

Keywords: co-referential expressions; intensional context; the law of identity; truth value

摘要： 同一性替换原则是经典逻辑的一个基本原则，然而它却遭遇了很多问题。弗雷格认为内涵语境的介入致使这一原则失效，罗素则以摹状词理论寻求其同一性悖论的解释，而维特根斯坦则从根本上否定这一原则，视之为伪命题。本文旨在论证同一性替换原则的有效性。讨论表明：从弗雷格到罗素再到维特根斯坦，他们对同一性问题的解释皆可证伪。其实，所谓的内涵语境是外在于同一性的，如果将内涵语境纳入一个命题，那么内涵语境就成了这个命题的一部分，只要不改变命题，即遵守同一律，那么同指的两个项可以替换而不改变真值。

关键词: 同一性, 内涵语境, 同一律, 真值