Racism

[Astonished]

Well, the difference in skin color was what made it so easy for them to differentiate. Finding and positively identifying escaped slaves of the same pigment as the slave masters isn't exactly feasible. So perception of race gave them a readily available means of both justifying (they're DIFFERENT from us! We can SEE the difference!) and keeping track (Hey, that person's darker than us...maybe they're an escaped slave!), at least in American slavery.

I agree with all that, except the part where you say finding and identifying slaves of the same pigment as the masters isn't feasible.   Throughout a lot of history slaves would have been the same race as the slave masters and I imagine they had methods such as branding, collars or/and chains to positively identify slaves.  That along with severe laws against being a runaway slave.

I agree that it would have made it easier to differentiate, but still wouldn't have been a cause of the slavery.

I imagine they were thinking about an opportunity to take vast plots of land, get free labor, get all the resources from the land.

On the other hand as a justification it seems to have been used.  The cause is the greed, but the justification of treating people like lesser humans was that they were racially inferior.  Which isn't always needed because of the countless examples of people having slaves the same race as them.

I'm not totally close minded about this but that's just my interpretation of the difference between the cause and justification.

Well, because the justification would have to be arbitrary, racial differences are as good as any.