The sense of body ownership concerns whether a body part or a whole body is experienced as belonging to me. For example, I am now typing this paper with two hands, and I have a sense that the two hands are mine. To clarify this concept of self-experience, three distinctions will be very useful. One is between the fact of body ownership and the sense of body ownership (Dokic 2003; de Vignemont 2011). The former is a biological fact about the anatomical structures of one’s body. The latter is a conscious experience of the fact of body ownership. As the syndrome of somatoparaphrenia indicates, these two aspects are dissociable. A prominent feature of somatoparaphrenia is that patients deny that parts of their body, e.g., a hand, belongs to them (Vallar & Ronchi 2009). Their sense of body ownership fails to match up with the facts —namely, that that the hand is theirs.
In healthy subjects, the sense of body ownership can also be mistaken. In the rubber hand illusion (RHI), participants experience a fake hand as belonging to them. The set-up is simple: The subject’s own hand is blocked from view. The subject sees a rubber hand in front of her, clearly distinct from her own real hand. The experimenter uses paint brushes to touch the real hand and the rubber hand either synchronously or asynchronously (Botvinick & Cohen 1998; Tsakiris & Haggard 2005). In the synchronous condition, many subjects report that they feel as though they are being touched on the rubber hand rather than on their real hand. More interestingly, many subjects feel as if the rubber hand were their own hand.[2]
Another form of misrepresentation involves the full body—an illusion that induces some interesting aspects of out-of-body experience (OBE) (Lenggenhager et al. 2007).[3] In experiments of this type, the subject wears a three-dimensional head-mounted display (HMD), and a stereo camera stands two meters behind her. The scenes registered by the camera are transmitted to the HMD such that the subject sees the back of his virtual body in front of her. Then the subject’s back is stroked either synchronously or asynchronously with the virtual body. In the synchronous condition, many subjects feel as if the virtual body were their own.[4]
The second distinction is between the first-personal sense and third-personal sense of body ownership. In daily experience, the sense of body ownership is often first-personal as well as pre-reflective (Legrand 2007, 2010). That is, by proprioception and somatosensation, I can experience the body as mine from the inside without watching it or reflecting upon it (de Vignemont 2012). Consider simple activities such as walking. When I talk to someone while walking, my attention can be fully absorbed in the conversation. In this case, I don’t pay any attention to my leg movements. Still, due to the firing patterns of muscle spindles in my legs, I implicitly experience that my legs take turns entering into the stance phase (touching the ground) and the swing phase (leaving the ground) to move my body forward. In contrast, the sense of body ownership can sometimes be third-personal and reflective. When looking at a monitor in an airport showing the image of my body, I may wonder whether the body that I see is mine. In this case, instead of experiencing it from the inside, I consider my body from the third-person point of view. That is, the body is treated as the object of visual experience, attention, or reflection.[5] In the rest of this paper, I will use “the sense of body ownership” to indicate the first-personal sense of the term.[6]
These two distinctions have been suggested before. But now I want to propose a third distinction to help elucidate what we mean when we talk about the sense of self-as-object. This third distinction refers to the difference between a sense of body ownership and a sense of self as a physical body.[7] The former relates to questions like “Is this my hand?” and “Is that body mine?”, whereas the latter concerns issues such as “What am I?” and “Am I a physical object?” This distinction marks two notions of bodily self-consciousness: experiencing a body part or a full body as one’s own, on the one hand, and being conscious of oneself as a physical body on the other. Conceptually, the sense of having a body and the sense of being a body are different notions.[8] However, they are closely related experientially. I suggest that experiencing ownership of a full body provides a sense of self as a physical body. When I engage in daily activities, there is not only a sense that this body is mine but also a sense that I am a physical body. Consider ordinary experiences like eating, running, bleeding, standing behind a desk, etc. These experiences involve a sense of body ownership, i.e. what it is like to have a body. But I also experience what it is like to be something that is eating, running, bleeding, etc. That is, I have a sense about what I am, or a sense of myself as a physical body that is doing these things.
I suggest that the sense of full-body ownership helps us to understand the sense of self-as-physical-body.[9] The sense of self-as-physical-body, in turn, helps us to specify what it means to be conscious of the self-as-object.[10] When I experience these hands as mine, there is a sense in which I am implicitly aware of myself as a physical body such that these two hands are parts of me. The proposal here is that I am conscious of myself-as-object when I am conscious of myself as a physical body. This holds not only in cases where I take myself as an object of vision or attention, such as seeing myself in a mirror. It holds even when I experience myself as a body from the first person perspective.[11]
Let me draw some remarks made by Wittgenstein to support this proposal. Consider his examples of “I”-as-object: “My arm is broken”, “I have grown six inches”, “I have a bump on my forehead” (1958, p. 67). These examples clearly refer to the speaker’s body. This fits my suggestion that consciousness of self-as-object can be understood as consciousness of self-as-physical-body—I have the sense that I am a body that has a broken arm or that has grown six inches. Now consider his examples of “I”-as-subject: “I see so-and-so”, “I try to lift my arm”, “I have toothache” (1958, pp. 66–67). As indicated by his own italicization, the use of “I”-as-subject is about who the perceiver, agent, or the subject is. But notice that these examples refer to the speaker’s body as well. What does this tell us? My interpretation is that it implies that the idea of who the subject is should not be regarded as the same as the idea of what does the perceiving, lifting, or undergoes toothache. The sense of self-as-subject is not equivalent to the sense of self-as-physical-body.
Towards the end of The Blue Book, Wittgenstein makes two important remarks. First, “we can perfectly well adopt the expression “this body feels pain”, and we shall then, just as usual, tell it to go to the doctor, to lie down, and even to remember that when the last time it had pains they were over in a day” (1958, p. 73).[12] His point is that we should not construe the thing that suffers pain as a Cartesian immaterial ego. The notion of body in the expression ‘this body feels pain’ can perfectly well refer to a physical object, i.e. to a person or to a biological organism that can consciously feel pain. Wittgenstein states this point from the third-person perspective. But there is no reason why this point cannot be formulated from the first-person perspective. That is, by “this body” I can refer to myself. As I suggested above, I can experience my body from the inside. Someone else can tell me to go to the doctor or to lie down, etc. In this case, I can be aware of myself as having a body that is in pain (a sense of body ownership), and I can have a sense of myself as a body that is in pain (the sense of self-as-physical-body).
This brings us to Wittgenstein’s second remark: “The kernel of our proposition that that which has pains or sees or thinks is of a mental nature is only that the word ‘I’ in ‘I have pains’ does not denote a particular body, for we can’t substitute for ‘I’ a description of a body” (1958, pp. 73–74). My interpretation of this remark is that, even when it is my body that is in pain, there remains a difference between saying “I have pains” and saying “this particular body feels pain”. When Wittgenstein says that “the word ‘I’ in ‘I have pains’ does not denote a particular body”, this remark can apply to the speaker’s body considered from the first-person perspective. The reason why we can’t substitute for “I” a description of a body is not because my body has to be described from the third-person point of view or that it has to be treated as an intentional object of consciousness. Rather, the reason we can’t substitute for “I” a description of a body is that the “I” in “I have pains” captures the sense of who feels pains, while “a particular body” captures the sense of what feels pains. This difference, then, marks two different types of self-consciousness. In the former case, I am conscious of myself as the subject of pain experience. In the latter case, I am conscious of myself as the body that feels pain. I do not mean that this is the only possible interpretation of Wittgenstein’s remarks. My claim is that it is a plausible interpretation, according to which the sense of self as subject of experience is distinct from the sense of self as a physical body, even when the body is characterized from the first-person perspective.
So far I have suggested an empirical approach to understanding the sense of self-as-object. The sense of full-body ownership provides theoretical and experiential grounds for understanding the sense of self-as-physical-body, which, in turn, helps to explicate the sense of self-as-object. This means that we can understand consciousness of self-as-object by studying the sense of full-body ownership. This fits Wittgenstein’s and Shoemaker’s assertions that the “I”-as object allows misrepresentation. The main advantage of my approach, however, lies in the fact that we know how to conduct empirical research on the sense of self-as-object. Now, in cognitive neuroscience there are plenty of exciting studies on full-body illusions and their neural mechanisms (Lenggenhager et al. 2007; Petkova & Ehrsson 2008; Ehrsson 2007; Ehrsson 2012; Ionta et al. 2011; Blanke 2012; Serino et al. 2013). A philosophical account will certainly benefit from looking at these. But what about the sense of self-as-subject? In the next section, I will appeal to the notion of experiential ownership in order to capture this basic form of self-consciousness.