[nightcracker]

Experimental science is all based around ruling out the null hypothesis. For that it needs to be falsifiable.

The hypothesis 'the dog can differentiate between A and B' is very hard to falsify. Because the dog could differentiate, but choose to not act. You would need a fairly complete understanding of the dogs mental workings, and scanners to study them.

On the other hand, 'the dog can't differentiate between A and B' is much easier to falsify. If you repeat an experiment a sufficient amount of times and the dog consistently has different behavior between A and B, you can rule this null hypothesis out.

That is the real reason we always 'assume the worst'. Because 'assume the worst' is the easiest to scientifically rule out.

EDIT: I would suggest this video by Veritasium, which also touches on this, at a very fundamental level: https://www.youtube.com/watch?v=vKA4w2O61Xo.

[Arech]

That's an excellent explanation!