A produces what he thinks B wants, but it may be a mistake, and B may not want it.

A may be able and willing to produce what B wants, but he may not be able to find B--he may not know of his existence.
The general truth of these principles is obvious, but what is not obvious is the extreme greatness of their effects.

Taken together, they make the whole difference between times of brisk trade and great prosperity, and times of stagnant trade and great adversity, so far as that prosperity and that adversity are real and not illusory.

If they are satisfied, everyone knows whom to work for, and what to make, and he can get immediately in exchange what he wants himself.

There is no idle labour and no sluggish capital in the whole community, and, in consequence, all which can be produced is produced, the effectiveness of human industry is augmented, and both kinds of producers--both capitalists and labourers--are much richer than usual, because the amount to be divided between them is also much greater than usual.
And there is a partnership in industries.