When does a brain transplant make something human?
Is a human brain in a vat human?
What about a human brain in a sheep?
Or a sheep brain in a human?
I assume that we couldnt transplant a human brain into a maple tree, because the latter lacks a nervous system. But I suppose life evolved from lifeless compounds, so in this sense can even inanimate material be human?
What do the answers to these questions say about what has the potential or is dictinctly human?
Before delving into the meat of the question, I would like to argue that, semantically, "humanity" is probably a misnomer for the condition you are trying to understand. It implies the importance of being a member of the human race, which is limiting to the larger question. Instead, "personhood" is the term I would suggest.
The core of this questions is the "Mind-Body Problem". There are two general sides to this discussion; Dualism, and Materialism.
Often attributed to Rene Descartes, mind-body dualism is the concept that there are two separate but interacting portions of our existence; the physical brain and the ethereal mind. To the dualist the mind is a quality in and of itself, and it interacts with the brain to manifest intellectual understandings. Opposing this view is materialism, which argues consciousness is completely dependent on the physical body, i.e. the brain and the mind is a manifestation of the brain.
I would refer you to Rene Descartes, Daniel Dennett, John Searle, and Thomas Nagel as some good sources to begin framing the two perspectives on the question. Here is a discussion with John Searle that discusses many of the foundational points and counterpoints to this debate. (LINK)
In response to your first set of questions...
Is a human brain in a vat human?
What about a human brain in a sheep?
Or a sheep brain in a human?
While the dualist and materialist do not agree on the origins of the mind, I believe that both hold agree that it is the existence of a mind that establishes personhood. Descarte described an animal brain as a clockwork mechanism devoid of a mind. Therefore, the sheep's brain in a human's body is an animal.
In the reversal, where a human brain in transplanted into a sheep, personhood could perhaps be transferred too. For a Dualists it would depend on if the mind is also transferred. For the Materialist, the mind necessarily transfers where the brain goes (if the brain maintains its functionality). This is also true for a literal "brain in the vat" scenario.
However, to ask if the mind could be "downloaded" into a computer, rather than transferring the brain to a vat frames the question slightly differently. It poses little issue to the dualist, since the mind is separate from the brain, and the computer would simply replace the mechanical portion of the current role of the brain. However, to a materialist, each mind is the result of a specific brain, therefore begs the question of how the same mind could exist outside the specific brain of its origins.
In response to your last question...
What do the answers to these questions say about what has the potential or is dictinctly human?
John Searle has an interesting thought experiment called the "Chinese Room", which he briefly discusses in the link provided above. Despite being a materialist, he makes the argument that simply making a machine that takes input and creates outputs that would mimic our thought process doesn't reach the level of understanding that is required to describe it as a mind. So, to your question the potential for person in any form is an ability to think and also understand.
Others, like Harry Frankfurt and Bennett Helm, focus on emotionality and choice as the key aspect in the attainment of personhood. Returning to the question of the sheep brain compared to the human brain, Helm argues that the ability have second-order desires and second-order volitions are the threshold that separates an animal mind from personhood.
First-Order Desire: a desire to do something; i.e. a desire to smoke.
Second-Order Desire: a desire to have a first order desire; i.e. a desire to quit smoking.
Second-Order Volition: the desire that one's first-order to be one's will, and to move one to act.
(Emotional Reason: Deliberation, Motivation, and the Nature of Value, Bennett Helm, 2001)
For an animal, there is only first-order desire. Given two desire, the one with the most pull will always win, despite any higher goals or considerations. Or in the case of equal desires, an animal such as "Buridan's ass" (LINK) would be forever stuck. However, Helm argues that a person capable of second-order volition would not be stuck. Instead they could call upon their ability for second-order volition to overcome the deadlock. Thus it is that criteria that must be met to establish personhood, as opposed to an animal mind.
Similar to your questions, another interesting permutation to evaluate is "if I take my brain and put it in your body... am I me, am I you, or are we a new person?"
More question arises from conditions like Dissociative Identity Disorder (previously known as Multiple Personality Disorder). I would think that for a materialist, no matter the number of personalities, a person with D.I.D. is one person. However to a dualist, I believe the question of whether each distinct personality is a distinct "mind" would be complicate the issue, and lea them to say that each personality is a different person.
There are two points I find important to answer this question:
1 - Physically speaking, a human is defined by more than its brain (two legs, two arms, one nose, the possibility of walking with the body in vertical position, DNA containing this and that, etc).
2 - We make judgments, ask questions and see meanings in things. These cannot be explained by neuro-chemistry because they cannot be physically codified the same way that feelings such as pain, attraction, sadness and so on can, so transplanting the brain alone won't make a sheep think like us. The implications of this is that we would need to transplant not only the brain but also our immaterial side to a sheep to make it "closer" to human.
Because of these two points, I think the answer to all the questions you asked is no.
The previous answers bring up some interesting philosophical literature, but it is not the literature that most directly relates to your question. So don't be deterred if the previous answers seem vaguely related to your question or insubstantial in sum. The philosophical literature that most directly relates to your problem is the area called personal identity, as someone will baseline understanding of the territory will testify.
In the area of personal identity, we ask questions about what makes a person a person, and what makes a person persist over time. Our primary method for investigating these questions is through thought experiments. Many of the thought experiments bring up scenarios like you describe. Is a brain in a vat a person? That's a good question.
To give you a taste of what the area of personal identity is like, consider this thought experiment. You enter into the teletransporter. You're told it works by scanning the position of every atom in your body and replicating it, atom for atom, using new matter, on a new planet. Your body on Earth will be disintegrated. You have misgiving about entering the teletransporter, but you decide to go through with it because many of your friends have gone through with it, and they seem fine. The teletransporter works. Your old body is disintegrated and a new body, thinking thoughts just like yours, is created on a faraway planet. Is this person you?
Now suppose the teletransporter makes two copies of you, on two different faraway planets. Are both of these people you? Is one of them you? Neither?
Many thought experiments like this one demonstrate that our ordinary concept of personhood might well fail to be coherent across the range of hypotheticals. Although a seasoned philosopher of personal identity might have very many interesting things to say about the particular hypotheticals you propose, the answer would likely end up indeterminate with respect to whether a person is present when, say, a brain is in a vat.
There is another question you can ask. Is an agent present? According to the standard story we tell in philosophy of action, an agent is a creature with beliefs and desires and the capacity to reason about how it should act in light of its beliefs and desires. However, given that we are dealing in hypotheticals, the very reasonable question "Is an agent present when a brain is in a vat?" will just depend on our stipulations about whether that brain, assuming it is conscious, has beliefs, desires, and interests.
Here is the SEP article about personal identity.
