If I see that you are about to leap off a cliff, I will advise you against doing that.

If you then tell me that the Land of Hilk and Money is on the other side and accuse me of depriving you, or that there are Starving! Children! you intend to succor and accuse me of indifference to their fate, I’m just going to call you a fool and continue to advise you against the jump, because you won’t get there and will hurt yourself in trying. I’m not telling you I won’t let you have it, I’m telling you you won’t get there.

Perhaps I have something at the bottom that would be negatively affected by the impact of 50+ kilograms of meat moving at terminal velocity for this atmosphere. If I dislike you personally, that might motivate me to warn you instead of letting you splash on the rocks — but, again, I’m not trying to say you aren’t permitted, I’m saying you won’t succeed. After all, if you did succeed my possessions would be safe from the impact.

When I tell you something won’t work, I mean exactly that, no more, no less — that is, it won’t achieve your stated goal, it won’t get you there. It does not mean I don’t believe the stated goal is desirable; in fact it makes no statement whatever about the goal. Shrieking that I don’t want you to reach the goal, when what I’m telling you is that the path you’re on won’t get to the goal, is jackassery of the first order — and leads me to suspect that you are lying, that your stated goal is not the same as your real one.