On Validities

In an Introduction to Symbolic Logic class offered by a philosophy class, you will probably learn:

  1. An argument is valid if when the premises are all true, the conclusion is (or must be) true.
  2. An argument is sound if it is valid and the premises are all true.

In such a class with a critical reasoning component, you will also learn about various common logical fallacies, that is, arguments which people take as valid but which are not (e.g., affirming the consequent, which is basically messing up modus ponens).

You might also get some discussion of “invalid but good” arguments, namely, various inductive arguments. (Perhaps these days texts include some proper statistical reasoning.) This notion is passé. I think reserving “validity” for “deductive validity” is unhelpful. In many scientific papers, there will be a section on “threats to validity” where the authors address various issues with the evidence they provide, typically:

  1. Internal validity (the degree to which the theory, experimental design, and results support concluding that there is a causal relationship between key correlated variables)
  2. External validity (the degree to which the theory, experimental design, and results generalise to other (experimental) populations and situations)
  3. Ecological (or field) validity (the degree to which the theory, experimental design, and results generalise to “real world” conditions)

There are dozens of other sorts of validity. Indeed, the Wikipedia article presents deductive validity as restricted:

It is generally accepted that the concept of scientific validity addresses the nature of reality and as such is an epistemological and philosophical issue as well as a question of measurement. The use of the term in logic is narrower, relating to the truth of inferences made from premises.

I like the general idea that a validity of an argument is the extent to which the argument achieve what it is trying to achieve.  Typically, this is to establish the truth (or likelihood) of a conclusion. Deductions are useful, but they aren’t what you need most of the time. Indeed, per usual, establishing the truth of the premises is critical! And we usually can’t fully determine the truth of the premises! So, we need to manage lots of kinds of evidence in lots of different ways.

An argument is a relationship between evidence and a claim. The case where the relationship is deductive is wonderful and exciting and fun, but let’s not oversell it.


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