Southern New Hampshire University
PHL-111: Introduction to Critical Thinking
Week Three Lecture Notes
It’s All About Transparency
The terms “validity” and “invalidity” apply to deductive arguments only. On the other
hand, inductive arguments are either “strong” or “weak”.
Why these distinctions? Because deductive arguments do not admit of degrees. There are
no “almost” or “nearly” valid arguments. A deductive argument is either valid or it isn’t. On the
other hand, inductive arguments do admit of degrees. The stronger the argument, the more
improbable the conclusion is false assuming the premises are true. The weaker the argument, the
greater the chances the conclusion does not follow from the premises as it claims.
Clarification of Terms
We use the term “strength” and correlatively “weakness” to describe the logical relationship that
subsists between the premises and the conclusion of an inductive argument – an argument whose
conclusion is more or less probable. We use the terms “validity” and “invalidity” to describe the
logical relationship that subsists between the premises and the conclusion of a deductive
argument – an argument whose conclusion follows necessarily.
Logical support has nothing to do with the truth-value of the propositions at issue. An inductive
argument can be strong while its premises and conclusion may be false. The same applies to
deductive arguments. The premise of a deductive argument may be false, and, hence, the
conclusion false while still being “valid.” Consider the two following arguments:
Here’s a strong inductive argument whose premises and conclusion is false:
P1: Most boys are aggressive.
P2: Samantha is a boy.
C: Therefore, Samantha is aggressive.
[This argument is strong albeit not cogent because the premises and conclusion are not actually
true. We assume the premises are true, however, and ask whether or not the conclusion is well-
supported to determine the strength or weakness of the argument. Had the premises and
conclusion been true, we would describe the argument as strong (because the premises make the
conclusion highly probable) and cogent (because the premises are true). Another thing to note
about inductive arguments: It’s possible to add a premise to make the argument stronger or
weaker, e.g., let’s add the premise:
P3: Samantha punched a classmate.
We could also make the argument weaker by adding the premise:
P3: Samantha helped a classmate.]
Here’s a valid deductive argument whose premises and conclusion are false:
P1: The moon is made of green cheese.
P2: Green cheese is edible.
C: Therefore, the moon is edible.
[Assuming the premises are true, the conclusion follows with logical necessity. Hence, it’s valid.
Were the premises actually true, we would describe this argument as both valid and sound. Here,
adding a premise has no effect on the argument’s validity. The conclusion logically follows no
matter what further information we add.]
