in a valid argument, if the premises are true, the conclusion must be true (the conclusion follows necessarily from the premises)

**If the premises can be true and the conclusion is false, then the argument is invalid!**

1. write argument in symbolic form on a line

2. fill in T/F columns under the variables

3. determine the T/F columns for all propositions

4. highlight or circle the column for each premise and conclusion

5. if any rows with T premise and F conclusion invalid (mark determining row(s)

1. write argument in symbolic form on a line

2. assume invalid by making all premises True and

conclusions False

3. work backwards, determining truth value for each operator and variable, avoiding contradiction if possible

4. if no contradiction, assumption is correct→invalid

5. if contradictionm assumption is incorrect→valid

Using a shorter Truth Table, when no more values can be determined with certainty:

1. guess (and mark) a value for one variable; fill in remaining values

2. if no contradiction→invalid (as assumed)

3. if contradiction, try opposite value for your guess. If still contradiction→valid, if not→invalid