In our class even inductive arguments will be considered a sub-type of invalid arguments. The major difference between these two types of arguments is explained in what follows.

Valid Arguments

If an argument is valid,
then it meets the following criteria:

If all the premises are true, then the conclusion must be true.

(In other words, the truth of the conclusion is guaranteed if all the premises are true)

OR

It is impossible to have a false conclusion if all the premises are true

OR

The premises of a valid argument entail the conclusion.

Conclusions deduced from a set of premises together with the premises themselves form a valid argument.

Here are some common examples of valid arguments::

Invalid Arguments

If all the premises are true, then the conclusion must be true.

(In other words, the truth of the conclusion is guaranteed if all the premises are true)

OR

It is impossible to have a false conclusion if all the premises are true

OR

The premises of a valid argument entail the conclusion.

Conclusions deduced from a set of premises together with the premises themselves form a valid argument.

Here are some common examples of valid arguments::

If John makes this
field goal, then the U of A will win. John makes the field goal . Therefore the U of A wins The Logical Name for this argument is Modus Ponens (this argument goes by other names as well, but this is the traditional name and the one used by Cohen and Copi in our textbook) |
The
general form of this argument is: If P then Q P Therefore Q |

If the patient has malaria, then
a blood test will indicate that his blood harbors at least one of these
parasites: P.
falciparum, P. vivax , P. ovale and P.
malariaBlood
test indicate that the patient harbors none of these parasitesTherefore the patient does not have malaria .The Logical Name for this argument is Modus Tollens |
The
general form of this argument is: If P then Q Not Q Therefore Not P |

Either The Patriots or the
Philadelphia Eagles will win the Superbowl The Patriots lost Therefore The Eagles won The Logical name for this argument is Disjunctive Syllogism, more commonly known as Process of Elimination |
The
general form of this argument is: Either P or Q Not P Therefore Q |

If John gets a raise, then he
will buy a house. If John buys a house, he will run for a position on the neighborhood council. Therefore, if John gets a raise, he will run for a position on the neighborhood council The logical name for this argument is Hypothetical Syllogism |
The
general form of this argument is: If P then Q If Q then R Therefore If P then R |

Invalid Arguments

If
an argument is invalid, then it
is possible for the conclusion
to be false even if all the premises
are true.

Invalid arguments come in all sorts of flavors, and students of Logic should be aware of the many different types.

One type of invalid argument is simply called a Logical Fallacy. These arguments are instances of pseudo-reasoning. The conclusion of a logical fallacy either does not depend on the truth of the premises at all (in such a case, we say the truth of the conclusion is independent of the truth of the premises) or the conclusion only follows very weakly from the premises. Unfortunately for those who are lovers of reason, logical fallacies are simply everywhere and one of the major goals of this class will be learning to recognize such fallacies when they occur.

Inductive arguments are another special case of invalid arguments - depending on the case, many inductive arguments have quite strong conclusions. Inductive arguments are not logical fallacies - since their conclusions are many times strongly inferred from the premises, however inductive arguments do not guarantee the truth of their conclusion, even if all of the premises are true (which makes them invalid).

WE WILL SAY that conclusion(s) arrived at by induction are strongly or weakly inferred from the premises.

The the conclusions of logical fallacies do not follow from the premises. By the way, "non-sequitor" is the Latin term used to describe conclusion(s) which do not follow from sets of premises!

Here are some examples:

Invalid arguments come in all sorts of flavors, and students of Logic should be aware of the many different types.

One type of invalid argument is simply called a Logical Fallacy. These arguments are instances of pseudo-reasoning. The conclusion of a logical fallacy either does not depend on the truth of the premises at all (in such a case, we say the truth of the conclusion is independent of the truth of the premises) or the conclusion only follows very weakly from the premises. Unfortunately for those who are lovers of reason, logical fallacies are simply everywhere and one of the major goals of this class will be learning to recognize such fallacies when they occur.

Inductive arguments are another special case of invalid arguments - depending on the case, many inductive arguments have quite strong conclusions. Inductive arguments are not logical fallacies - since their conclusions are many times strongly inferred from the premises, however inductive arguments do not guarantee the truth of their conclusion, even if all of the premises are true (which makes them invalid).

WE WILL SAY that conclusion(s) arrived at by induction are strongly or weakly inferred from the premises.

The the conclusions of logical fallacies do not follow from the premises. By the way, "non-sequitor" is the Latin term used to describe conclusion(s) which do not follow from sets of premises!

Here are some examples:

Logical Fallacies

I have always liked Michael J.
Fox, and now his battle with Parkinson's disease is really sobering. He certainly is a man acquainted with grief. He is also a vegetarian, therefore not eating meat is probably not a good idea. |
The conclusion is that one
should not be a vegetarian, which seems to take its strength from the
fact that Michael J. Fox is now not healthy. In other words, there is
an innuendo (which is disguised by the first statement which states a
personal like toward Michael J. Fox) that tries to connect Parkinson's
disease with being a vegetarian. In other words, this is an example of
false cause and hasty generalization. Since no causal links
between vegetarianism and Parkinson's disease have been stated, and
from one case you can not generalize to other cases. |

The Powerball has reached a
near-record jackpot of $210 million dollars. Almost anyone would like
that kind of money, and one thing is for sure, if you don't play, you
can't win. Therefore Play Powerball! |
In this case the conclusion is
that one should play Powerball. The reason for this conclusion seems to
follow from three true premises. 1) The Jackpot has reached a
near-record high. 2) Almost anyone would like that kind of money and 3)
You can't win if you don't play. However, there is an additional unstated true premise which makes the conclusion very weak, specifically that the odds of wining the powerball are one chance in 120,526,770. This by definition is extremely improbable! (Go here to see how this figure was calculated) |

Inductive Arguments

Every Banana plant that I have
grown outside always dies immediately at the first touch of frost. Therefore, the banana plant growing outside will die too when we get our first frost. |
The conclusion to this argument
certainly is not guaranteed, even if the premise is true. The strength
of the conclusion increases with the number of banana plants the person
has grown, and also knowing that no other important fact about banana
plants has changed (such as genetic variants which enable them to
survive below freezing temperatures) |

I have always owned Ford
vehicles, and have always been pleased with their performance and
reliability - therefore I should buy another Ford this time too. |
Again, the same considerations
listed above apply to this conclusion as well. If the person had only
owned one Ford in his life, the conclusion would be weak. If the person
had owned several Fords, then the conclusion does seem to be somewhat
strong (certainly many other factors need to be considered before
coming down on the side of just how strong the conclusion really is) -
but this argument, like many inductive arguments, argues from past
experience to future expectations - which is nicely illustrated in the
next argument paraphrased from the Philosopher David Hume. |

I have eaten toast with butter
an jam every morning for the most of my life. Therefore I may eat toast with butter and jam this morning, and it will not poison me. (The toast I ate yesterday will not poison me today!) |
Again, this argument is
inductive, and most would say the conclusion is strongly inferred from
the premises. Of course additional information may change things (NOTE:
To state that the servant poisoned the toast to kill the master does
not necessarily change the argument's conclusion, since in this case it
is neither the toast nor the jam that kills the master, but rather the
poison placed in it!) |

FINAL NOTE:

Ways to tell the two types of arguments apart!

Ways to tell the two types of arguments apart!

FOR VALID arguments, the addition of
extra premises can not change the conclusion - a valid conclusion
deduced from a set of premises can never be changed by the addition of
new premises.

Also, it is inconceivable for the premises of a valid argument to be true and the conclusion to be false (just try it!)

FOR INVALID arguments, the addition of new premises will many times strengthen or weaken a given conclusion.

Also, it is conceivable for the conclusion of an invalid argument to be false even if it does have true premises!

Also, it is inconceivable for the premises of a valid argument to be true and the conclusion to be false (just try it!)

FOR INVALID arguments, the addition of new premises will many times strengthen or weaken a given conclusion.

Also, it is conceivable for the conclusion of an invalid argument to be false even if it does have true premises!