I love your video. I'm taking a philosophy course and I was so confused on what the meaning of valid and invalid deductive reasoning meant , until I seen this video. You explained it a whole lot better. Thank you.
I like to think of the premises & conclusions in syllogisms in terms of Venn diagrams. Example 1: Premise 1: "All actors are robots" (draw circle labeled 'actors', and shade 100% of it to indicate all it as robot-filled) Premise 2:"Tom Cruise is an actor" (given we're told Tom Cruise is an actor, find his place in circle) Conclusion: "Tom Cruise is a robot" (upon looking at the 'actor' circle, which is 100% robot shaded, it's logical to conclude Tom's secret identity) Example 2: Premise 1: "All actors are robots" (draw circle labeled 'robots', and inside shade a small area for 'actors') Premise 2:"Tom Cruise is a robot" (given we're told Tom Cruise is a robot, find his place in circle) Conclusion: "Tom Cruise is an actor" (upon looking at the 'robot' circle, which only has a part shaded for actors, it would NOT be logical to definitively conclude Tom is an actor, when there exist possibility he could be something else)
OK, I don't get this. You say at 1:47 that "IF all the premises are true, then the conclusion CANNOT be false." Then you say, "IF all the premises are true, then the conclusion CAN be false." Which one is it?
in categorical syllogism simply look for the three categories in their proper place. (category 1) (category 2) (category 3) (category 1) (category 3) (category 2) example (c1 robots) are (c2 actors) (c3 tom cruise) is a (c1 robot) (c3 tom cruise) is an (c2 actor) c1 = robot c2 = actor c3 = tom cruise example all gods are mortals men are gods men are mortals or all men are gods mortals are men mortals are gods or mortals are men gods are mortals gods are men all these are valid arguments as long as the categories are in their right places.
Ok, here's what I DON"T get! If the first example says that All actors are robots and Tom Cruise is an Actor therefore he's a robot, we know is actually not true, yet we assume it is in the context of this argument... so why can we not assume similar things in the second argument where it might be that 'All Actors a robots'... I mean we know that its not true in both cases yet the example in the second one says invalid... that makes no sense to me.
On the second example, it is also assumed that all actors are robots, to evaluate its validity. The problem is that,different from the first argument, the 2nd premise of the 2nd argument, states that Tom cruise is a robot (not an actor), so even if the two premises were true, the conclusion would not follow. For instance, it could be that Tom cruise was not an actor , but a bell boy , and yet a robot. In the 1st argument such exception does not exist, if all actors are robots and Tom Cruise is an actor, then, he must be a robot. Hope that helps.
Sorry to resurrect your thread, but that's just the kind of guy I am. 8) Strongness is a feature of contingent/inductive arguments, and is comparable to the validity check of a deductive argument, in that we are trying to determine how well the premesis link to the conclusion (assuming for now the premesis are true). For now we're just checking the form of the argument and aren't too concerned about whether individual terms really are true or false. Knowing the formal fallacies can help at this juncture. If we aren't using fuzzy terms like 'most', 'some', '75%', and so on in our argument, then we check the arguments validity by seeing if all the premesis evaluate to true and the conclusion evaluates to true, using boolean logic on the atomic/molecular terms. If it meets this check, it's deductive/valid/tautology and conclusion is guaranteed *not to be flawed by the form of the argument*; if not, it's invalid (read: conclusion not guaranteed) and inductive (provided it's not a logical self contradiction). If it's inductive you have to gauge the strength or weakness of the argument using the fuzzy terms (or mountains of scientific data, whatever floats yer boat). At this point if we're confident in the reliability of the form of the argument, we check the premesis to see how good the truth values measure up to reality. Knowledge of the informal fallacies helps here. If it's your own argument, you can go back and check the arguments that gave you the premesis for your main argument; if not, guesstimate their worth. If it passes muster, then it's called *sound* if it's the deductive variety, or *cogent* if of the inductive sort; 'unsound' or 'uncogent' if something smells fishy. There's some good videos on methods to check validity. Search for 'Truth Trees' or 'Reductio Proof'
What you do not understand is if a equals b and b equals c, a does not equal c. Valid arguments are not valid. Truth is not always what people have been taught and some believe all valid arguments are valid simply because they were taught that.
