There is a poll over at Gizmodo asking readers What Percentage of Our Body Would Have To Be Replaced Before We Ceased Being Human?

The options that the poll offers are mostly percentages — 10 percent, 20 percent, and so on — which is pretty silly, since it suggests that percentages are a useful way of talking about the human organism. (What is “20 percent” of a human body? Is that by mass? Or volume? Or perhaps surface area?) The logic of the poll’s options leads the respondent to think about the question along the lines of the Sorites paradox: “Well,” a respondent might think, “I don’t see what the big difference would be between 30 percent and 40 percent… or between 40 and 50 percent… or between 50 and 60…”

Given those options, it’s no surprise that three quarters of the respondents (as of this writing) have instead picked the following choice: “You can take away nearly everything, but if our brains are replaced by machines, we cease being human.”

In the comments beneath the poll, some readers objected to the options they were given. Commenter “newgalactic” argues that the correct answer to the question is a figure lower than any among the poll’s options: “The ‘body,’” he writes, “has more ties to the ‘mind/soul’ than we realize.” (He also wonders whether the poll results might be skewed by the fact that most Gizmodo readers are “dorks/nerds” who are “less physically blessed,” while someone with a body like Brad Pitt might “be more inclined to attach his humanity to his physical body.”)

The comments also suggest that any attempt to take the poll’s silly question seriously must start by asking a deeper question: What does it mean to be human? In the first issue of The New Atlantis, bioethicist Gilbert Meilaender described some of the difficulties that the deeper question entails:

We might try to think of human beings (or the other animals) [as collections of parts], and, indeed, we are often invited to think of them as collections of genes (or as collections of organs possibly available for transplant), but we might also wonder whether doing so loses a sense of ourselves as integrated, organic wholes.

Even if we think of the human being as an integrated organism, the nature of its unity remains puzzling in a second way. The seeming duality of person and body has played a significant role in bioethics. As the language of “personhood” gradually came to prominence in bioethical reflection, attention has often been directed to circumstances in which the duality of body and person seems pronounced. Suppose a child is born who, throughout his life, will be profoundly retarded. Or suppose an elderly woman has now become severely demented. Suppose because of trauma a person lapses into a permanent vegetative state. How shall we describe such human beings? Is it best to say that they are no longer persons? Or is it more revealing to describe them as severely disabled persons? Similar questions arise with embryos and fetuses. Are they human organisms that have not yet attained personhood? Or are they the weakest and most vulnerable of human beings?

Related questions arise when we think of conditions often, but controversially, regarded as disabilities…. Notice that the harder we press such views the less significant becomes any normative human form. A head, or a brain, might be sufficient, if it could find ways to carry out at a high level the functions important to our life.

Such puzzles are inherent in the human condition, and they are sufficiently puzzling that we may struggle to find the right language in which to discuss that aspect of the human being which cannot be reduced to body. Within the unity of the human being a duality remains, and I will here use the language of “spirit” to gesture toward it. As embodied spirits (or inspirited bodies) we stand at the juncture of nature and spirit, tempted by reductionisms of various sorts. We have no access to the spirit — the person — apart from the body, which is the locus of personal presence; yet, we are deeply ill at ease in the presence of a living human body from which all that is personal seems absent. It is fair to say, I think, that, in reflecting upon the duality of our nature, we have traditionally given a kind of primacy to the living human body. Thus, uneasy as we might be with the living body from which the person seems absent, we would be very reluctant indeed to bury that body while its heart still beat.

A definition of human being based only on biological parts will fail to capture the unique nature of the living human. A definition based on biological functions will fail to include human beings who lack those specific functions. Indeed, any strictly biological definition will miss the qualitative aspects of what it means to be human — how we live and behave over the course of our lives; what we do and are capable of doing; what we feel and experience; and how all of it changes — in short, the phenomena of life. And so a rich understanding of what it means to be human might start with science but must go beyond it, seeking wisdom especially in the disciplines rightly called “the humanities.”

There can be no honest answer to the Gizmodo poll as it is phrased, and there is no easy answer to the deeper question of what it means to be human. But the search is rewarding — and, in a way, the search may itself be part of the answer.

(h/t Instapundit)