The human element. Whether or not that which is spoken of is, in fact, human at all. The concept of personhood brings with it an inherent aspect of randomness, at least on the cosmic scale of things. It could be argued that because a person is the creation of their lives, their upbringing, their beliefs and fears and hopes and dreams, that there is no true randomness to their actions. That their choice is but the illusion of choice, that two identical people will make the same decision when presented with identical situations. Their decision being guided by the sum of their lives up to that point. If the person you are controls all your choices, and your experiences determine the person you are, then your experiences control what choice you make. Logically speaking, it is a valid interpretation, but what then of free will?
There is the belief in many cultures and religions that humanity is the only living beings on the world to have free will. That they alone among the myriad of creatures can truly choose what they do. That they are not controlled by instinct or nature, but are instead guided by it while the actual choice of decisions is made by the higher brain. This is see most blatantly in the work of Freud, encompassed within the structure of id, ego and superego. The debate still rages today as to what classifies personhood, where and when does sentience begin. Can a line fine enough be drawn so that we can definitively say that those who stand on one side of it are sentient beings and those on the other side are not? Is there terminology sufficient to this magnum opus? There was a time when awareness was the benchmark for this, and yet animals have awareness, some not as great as humanity, and yet others have awareness far greater than we will ever know.
More importantly why are we asking the question in the first place? Does the answer truly matter? Do we fear finding out that we do not stand on the side of the line that we thought we should? Do we fear learning that those we have dismissed as non-sentient have been wronged by us, and all the consequences that entails? Are we uncertain of what the answer to our questions will even entail, and thus dread that unknown? The answer is yes...and no...and also maybe. Humanity, on the whole, is complicated and nuanced and it would require vivisection on the individual level to truly understand a person, metaphorically speaking of course. So let us take a close look at certain individuals and see what they might tell us about themselves.