# How do you know two premises are combined to support an argument?

forgive me if I'm asking a simple question, but I'm trying to learn Introduction to Logic by Irving M Copi. I'm trying to learn how to analyse arguments. One of these arguments he uses as an example to show that each proposition supports the conclusion in a combined way:

General Motors makes money (when it does) on new cars and on the financing of loans. Car dealers by contrast make most of their money on servicing old cars and selling used ones. So car dealers can thrive even when the automaker languishes.

My question is, how does one know that premises are to be combined in the first place in order to support a conclusion in general?

• The middle term connects the two premises. Is that what you wanted to know? Nov 27, 2023 at 2:54
• Well you just have to understand the argument being made. In this case, they're saying that because automakers and car dealers make money in different ways, there can be a good market for one at the same time there is a poor market for the other. If you understand the argument, it should be clear which statements are supporting others. It's kind of intuitive, or rather it can't really be explained. You just look at it until you understand how the argument hangs together. Nov 27, 2023 at 7:43
• Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. Nov 27, 2023 at 8:19
• "combined"? Using inference rules, i.e. "patterns" that instruct us to manage compound proposition, like e.g. Modus Ponens: "form if A then B and A, infer B" and Syllogism: "from if A then B and if B then C, infer if A then C". Nov 27, 2023 at 8:30
• If a fight breaks out between the residents of adjacent semis owing to the appalling racket coming through the party wall, then you know two premises are combined to support an argument. Nov 27, 2023 at 22:43

It's because each premise relates to a piece of the conclusion. In formal logic it's easier to see it, because it becomes very mathematical and precise, but even in informal logic like this, you can see the overall pattern.

We can write it in pseudo-formal notation to make it easier to see.

A) General Motors makes money from new cars
B) Car dealers make money (primarily) from used cars
Therefore (C) B can happen even if A doesn't

That's NOT a strict formal argument, but we can see how both B and A are essential parts of the conclusion.

The brain is a logical organ, so this is essentially an intuitive process.

You consider what the conclusion is, you consider what it means, you think about it as long as necessary, and hopefully, it will dawn on you that these or those premises would if true make the conclusion necessary.

If you are already given the premises, as in Copi's book, you go through the same procedure only it should be much easier.