argument from analogy (idea)

A form of inductive argument.

1. X has characteristics/properties P1, P2, ..., Pn.
2. Y has characteristics/properties P1, P2, ..., Pn.
3. X has characteristics/properties Z
4. There are no (or very few) significant and relevant differences between X and Y.
5. Therefore, Y has characteristics/properties Z

To illustrate:

1. My car has four wheels, airbags, fuzzy dice, bucket seats, trash on the floor, four doors, an antenna 3 ft. long, and was made by Ford.
2. Alice's car has four wheels, airbags, fuzzy dice, bucket seats, trash on the floor, four doors, an antenna 3 ft. long, and was made by Ford.
3. My car is red.
4. I know of no relevant differences between my car and Alice's car.
5. Therefore, Alice's car is red.

As you can see, it doesn't always work out in real life (in case you didn't know it, Alice's car is blue).

The more similarities, and the fewer (relevant) differences, the more certain the conclusion is. Beware! Sometimes people will try to leave out point #4. This should be cause for great suspicion.