What is David Ricardo's comparative cost theory of international trade?

The theory states that even if a country can produce both commodities more than the other there is still the need to specialze in the production of a particular commodity of which she has the lowest cost advantage and exchange it for the other, for trade between countries to be mutually benefitial."all things being equal".

ASSUMPTIONS:

1- there are only two countries in the world.

2- there are only two commodities.

3- trade between two nations are not restricted.

by; ICE