It is one thing to use a mental state; it is another to have a clear comprehension of just what it is and of what elements it may be made up.

The plain man does much of his thinking as we all tie our shoes and button our buttons.

It would be difficult for us to describe these operations, but we may perform them very easily nevertheless.

When we say that we _know_ how to tie our shoes, we only mean that we can tie them.
Now, enough has been said in the preceding sections to make clear that the vagueness which characterizes many notions which constantly recur in common thought is not wholly dispelled by the study of the several sciences.

The man of science, like the plain man, may be able to use very well for certain purposes concepts which he is not able to analyze satisfactorily.