Of course, in real life the pear tree and the cherry tree have plenty of differences, even if we are unable to perceive them. The issue I was hinting at was the following: What enables a trained person to select those properties which distinguish a pear tree from a cherry tree inspite of the fact that there are also plenty of differences between individual cherry trees. Or look at the variety of animals we all call "dog". (of course you will say we know them by the bark).Mental properties, as I would define them, apply to classes of objects which we have named by a common term. Such classes are not physically given, but even culture-dependent.