Analogy

An analogy is when you point out the similarities between two different things.

If I say “the mind is like a computer: it takes certain inputs, processes them, and then spits out results”, I’m drawing an analogy – I’m saying that these two different things – the mind and a computer – are actually alike in some way. But I’m not giving an argument – I’m not drawing any conclusion from my analogy. I’m just drawing parallels, maybe trying to make you think about the mind in a different way, illustrating something about how the mind works.

An argument by analogy is when you point out the similarities between two or more things and then draw some conclusion.

Suppose there is a particular car that I’m deciding whether or not to buy. It’s a ten-year-old Honda Civic with 75,000 kms on the clock, it has a little rust but not much, and it’s only had one owner and she drove it carefully and got it serviced regularly.

And suppose I reason like this:

My last car was a Honda Civic, and when I bought it it was 10 years old had 75,000 kms on the clock, it has a little rust but not much, it had had only one previous owner, and that owner had looked after it well. And my last car served me well – I drove it for 5 years and it gave me no trouble and needed very little in the way of repairs. So this car, being so similar to my last, will probably likewise serve me well – I should buy it.

P1. Car A was a Honda Civic, ten years old, 75,000 kms, only a little rust, well looked after, and it served me well for 5 years each and required very little in the way of repairs.
P2. Car B is a Honda Civic, ten years old, 75,000 kms, only a little rust, well looked after.
                                        
C. Car B will last 5 years and hardly need any repairs.

General structure of arguments by analogy:

A has characteristics W, X, Y and Z
B has characteristics W, X, and Y
                                         
B will have characteristic Z as well.

Things to consider in evaluating an argument by analogy:

• How similar are the things being compared?
• Are the similarities relevant?  (e.g. if the similarities mentioned were all things like colour, which is surely irrelevant to how well the car goes, it would not be a good argument.)
• Are there any relevant differences between the things being compared?
• How many similar cases are we dealing with?  (e.g. If I had had 3 cars which were similar in all of these respects and had all served me well, the argument would be stronger.)