Another way to look at it - a plane on water.

The water is most certainly not solid. Yet the plane takes off (if it has enough power to get up to speed).

The plane may take longer to take off going against current as opposed to with the current - but it will eventually.

I think...

My brain hurts. It's been WAY too long since I bothered with this stuff.
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"Nature has constituted utility to man the standard and test of virtue. Men living in different countries, under different circumstances, different habits and regimens, may have different utilities; the same act, therefore, may be useful and consequently virtuous in one country which is injurious and vicious in another differently circumstanced" - Thomas Jefferson, moral relativist