Chisholm wanted to say that we can have knowledge of things beyond those we “see with our own eyes”. These would include things that are confirmed by the evidence of our senses. As a step toward achieving this, he has a principle about such things. The principle makes use of the concept of the empirically acceptable. To find what’s empirically acceptable for S, just consider all those propositions that get a rating of acceptable (or higher) by the aforementioned principles. Those are empirically acceptable for S at t. Chisholm does not define confirmation, but assumes we understand it.