Please sign your comments The modal fallacy is already explained in the article Formally stated the modal fallacy is as follows 1 It is necessary that if A is true then B is true 2 A is true 3 Therefore it is necessary that B is true The modal fallacy occurs when someone takes a necessary truth if A is true then B is true and then applies that necessity to one of the individual parts B is true Here is an example that illustrates this 1 If Dave has two sons then Dave has to have at least one son 2 Dave has two sons 3 Therefore Dave has to have at least one son It is necessary that Dave has at least one son This proof is incorrect since it is not logically necessary that Dave has at least one son He could have chosen to have no children or he could have had all daughters Just because something is true that does not make it necessarily true More properly stated the proof goes like this 1 It is necessary that if Dave has two sons then Dave has at least one son 2 Dave has two sons 3 Therefore Dave has at least one son When the proof is phrased properly and the modal fallacy does not occur then the statement Dave has at least one son is shown to be true but not necessarily true So if you apply this to the argument from free will the proof is as follows 1 It is necessary that if God knows that a person will do action A at time T then that person will do action A at time T 2 God knows that a person will do action A at time T 3 Therefore the person will do action A at time T The modal fallacy would be changing the last statement to Therefore it is necessary that the person will do action A at time T Concluding that someone will do something is very different from concluding that they must do something I really don t know how to explain it any better