And-Introduction:

In generally English says that "if we know that a lot of things are true, so we know that the conjunction of all of them is true", then we can introduce conjunction 'and' symbols. 

(A1,   A2,   ...,    An ) ÷ (A1 ? A2 ? ... ? An )

However this may not seem to be saying more. In fact, by imagine that we are working with a bunch of different sentences at different places in our knowledge base so here we know some of them are true. Thus we can make a larger sentence out of them by conjoining the smaller ones.