Logical Arguments
A logical argument has three basic parts: premises, inferences, and conclusions.
Parts of an Argument
Premises
A premise is a statement that is accepted as true. The person making the argument does not try to prove it but simply states it as a "given." You can identify a premise by imagining the word “if” in front of it.
Premise 1: (If) People with pets have lower levels of depression than those without pets.
Premise 2: (If) The companionship of a pet reduces loneliness.
Imagining the word “if” in front of a premise helps you remember that the writer is saying, “Assuming that . . .” It also shows that other statements are needed to complete the thought.
Note that if a premise is false, the whole argument may be false. If studies do not show significantly lower depression for people with pets, the argument is already on shaky ground.
Inferences
An inference is a statement that is derived from previous statements—whether premises or other inferences.
Premise 1: (If) People with pets have lower levels of depression than those without pets.
Premise 2: (If) The companionship of a pet reduces loneliness.
Inference 1: (Then) Caring for a pet offers psychological benefits.
Premise 3: (If) People with dogs tend to take more walks than those without dogs.
Inference 2: (Then) Having a dog helps mind and body.
The first inference derives from premises 1 and 2, and the second inference derives from inference 1 and premise 3. Imagining the word “then” in front of inferences helps you remember that they depend on previous statements. An inference is true only if it is properly derived from previous statements that also are true.
Conclusions
A conclusion is a final inference, derived from the previous statements or inferences. Identify a conclusion by imagining the word “Then” in front of the final inference.
Premise 1: (If) People with pets have lower levels of depression than those without pets.
Premise 2: (If) The companionship of a pet reduces loneliness.
Inference 1: (Then) Caring for a pet offers psychological benefits.
Premise 3: (If) People with dogs tend to take more walks than those without dogs.
Inference 2: (Then) Having a dog helps mind and body.
Conclusion: (Then) Adopting a dog will help me improve my mental and physical health.
Note that this conclusion is properly derived, but is it true? Only time will tell. Selecting a loving and well-behaved dog quite likely will have the desired effect. Selecting a vicious and destructive dog probably would not.
Hidden Assumptions
A hidden assumption is an unstated premise that may or may not be true. For example, premise 1 assumes that pet ownership is the cause of lower depression rather than the effect of it. Perhaps depressed people don't have the energy to care for pets and so don't adopt them. Premise 2 assumes all pets are good companions. It also ignores social isolation from family and friends who are allergic and no longer come over.
Just because an argument has hidden assumptions doesn't mean the argument is wrong, though. In fact, all arguments contain hidden assumptions. It would simply take too long to state everything that most people would accept as true.
Hidden Assumption: (If) Children are not pets.
Hidden Assumption: (If) Plants are not pets.
Hidden Assumption: (If) Inanimate objects are not pets.
These hidden assumptions are true and go without saying. They do not impact the overall strength of the argument.
Testing Arguments
When you build an argument, you should test every premise to make sure it is true, and test every inference and conclusion to make sure it is properly derived and true. You should also identify hidden assumptions and make sure they are true.
When you consider another person's argument, you should test these same parts. If you find a false premise or assumption, challenge it. If you find a poorly derived inference or conclusion, question it. Ask yourself, "Is this true in all cases, most, some, or none?"