What is the ruling on sitting on couches brought from a man who raises a dog, when one's pants are wet?

The couches that were in the possession of a dog owner are, in origin, pure (tahir). They are not to be ruled as impure (najis) unless we are certain that impurity has afflicted them. Doubt does not remove them from their original state of purity. The mere act of keeping a dog does not necessitate their impurity. Therefore, there is no harm in sitting on them, even if what touches them is wet.