Part four: using 'must' to talk about logical necessity.

第四部分：用“必须” 谈逻辑必然性。

Now, since these facts of experience do not follow of logical necessity, one must suppose that the environment and the psychological propensities of the modern world must be of such a character as to produce these results.

现在，由于这些经验事实并非逻辑必然性，因此我们必须假设现代世界的环境和心理倾向一定具有产生这些结果的性质。