How could we have knowledge of moral properties?

Granting, at least for the sake of argument, that there are moral truths, you might nevertheless think that there's no point doing political philosophy because no one can ever know those truths.

I take it that your scepticism about moral knowledge here isn't based on a general scepticism about the possibility of any knowledge. A discussion of general scepticism would take us too far from the point here. About that, we can just say that it takes argument to justify a general scepticism of this sort. You can't just assert it and be done.

So, I'll assume that your scepticism is based on something about moral truths in particular.

Now, there are troubling questions for moral realists about how it is that we come to know the moral facts that realists suppose there are. It doesn't seem as if we see moral facts in the way that we see bottles and trees and people, for example. And it's a bit mysterious to posit instead special 'moral sense' by which we simply intuit moral truth.

However, realists aren't at a complete loss when it comes to moral knowledge. Some of them, for example, identify moral properties with (collections of) physical properties. These realists think that so long as we can have knowledge of the physical properties (and surely we can have knowledge of many physical properties just by looking at them, for instance), we can thereby have knowledge of the moral properties that are identical with those physical properties.

Other realists think that moral properties 'supervene upon'—or 'come along with', as we might put it more colloquially—physical properties, even though they're not identical with them. For these realists, we judge that the moral properties are there based upon our ability to see (or touch or feel or hear) the physical properties that the moral properties supervene upon.

There are other strategies besides these. Not all of them will seem plausible to you, but I hope this gives you an indication that you'll need more than just a kind of incredulity about the possibility of moral knowledge in order to justify scepticism about the point of doing moral and political philosophy.