Assuming that I am human, and my peers are human as well, then it follows that we share something in common. We all seem to share in common the property (humanness), just as a lemon would share the property yellowness with another lemon. But let us look closer at the relationship between lemons for a second. Provided of course that the lemons in question are good and ripe they will have said commonality. So then, this lemon and that lemon are yellow. The shirt in my closet is yellow; as are the little sticky pads I have on my desk. All these objects are equal, in that they are equally yellow (they may be different shades, but this makes no difference as they are still shades of yellow). If we are to agree that provided one thing is in common to another thing, then it cannot be the case that in the same respect to which these things are in common they can also be different. So as far as yellow is concerned, these things are not different from one another. Yet at the same time we see these things as individual things and as such they must be different from one another. We're left with individual things that cannot be equal to one another. So what of the equality that we know to be there? The equality of yellowness must be distinct from the individual things; it must be another thing.
This other would be considered a universal, and if we take the analogy to be a good one, then humanness would have to be a universal as well. For I am a human, just as Helder is, and we share in the commonality humanness. But remember in said analogy I had granted that two lemons, so long as they were both ripe, would share the universal- yellowness. If it were the case that one lemon was quite rotten, then the two lemons probably wouldn't have this commonality. So then would there be a situation in which two humans wouldn't share in humanness?.
Consider a baby born sans a functioning brain. So long as we are speaking of the offspring of the human reproductive cells, or even chemistry (what I will refer to as a baby), then yes, this baby is a human.
