Key Arguments

• Parfit cites clinical neurosurgery for epilepsy in which 'surgeons have cut' the fibres connecting the hemispheres, reporting that 'the effect, in the words of one surgeon, was the creation of “two separate spheres of consciousness”', indicating distinct concurrent experiential centers in one body.
• Psychological tests exploit contralateral control of limbs and visual fields so that 'psychologists can thus present to this person two different written questions in the two halves of his visual field, and can receive two different answers written by this person’s two hands', demonstrating two semi‑independent centres processing different information.
• In a stylized test, the subject sees a half‑red, half‑blue screen, initially writes with both hands 'Only one' in answer to 'How many colours can you see?', then when asked 'Which is the only colour that you can see?' one hand writes 'Red' and the other 'Blue', showing that 'in seeing red he is not aware of seeing blue, and vice versa.'
• Parfit generalizes from the imagined to the actual tests: 'In seeing what is in the left half of his visual field, such a person is quite unaware of what he is now seeing in the right half of his visual field, and vice versa. And in the centre of consciousness in which he sees the left half of his visual field, and is aware of what he is doing with his left hand, this person is quite unaware of what he is doing with his right hand, and vice versa.'
• He notes further conflict phenomena—e.g. the pipe/pencil case where the hemispheres alternatingly control the same hand, and the report that 'sometimes, when he embraced his wife, his left hand pushed her away'—to illustrate that distinct motivational and perceptual centres can coexist and compete within one biological person.
• On a Reductionist view, these facts show that the 'unity of consciousness' can itself come in multiple, simultaneous instances within one life, supporting the claim that personal identity just consists in (and can branch into) various physical and psychological continuities, rather than requiring a single, indivisible subject.

Source Quotes

87. DIVIDED MINDS SOME recent medical cases provide striking evidence in favour of the Reductionist View. Human beings have a lower brain and two upper hemispheres, which are connected by a bundle of fibres.

But the operations had another unintended consequence. The effect, in the words of one surgeon, was the creation of ‘two separate spheres of consciousness’.33 This effect was revealed by various psychological tests. These made use of two facts.

And what is in the right halves of our visual fields we see with our left hemispheres, and vice versa. When someone’s hemispheres have been disconnected, psychologists can thus present to this person two different written questions in the two halves of his visual field, and can receive two different answers written by this person’s two hands. Here is a simplified version of the kind of evidence that such tests provide.

On each half in a darker shade are the words, ‘How many colours can you see?’ With both hands the person writes, ‘Only one’. The words are now changed to read, ‘Which is the only colour that you can see?’

The words are now changed to read, ‘Which is the only colour that you can see?’ With one of his hands the person writes ‘Red’, with the other he writes ‘Blue’. If this is how this person responds, there seems no reason to doubt that he is having visual sensations—that he does, as he claims, see both red and blue.

The many actual tests, though differing in details from the imagined test that I have just described, show the same two essential features. In seeing what is in the left half of his visual field, such a person is quite unaware of what he is now seeing in the right half of his visual field, and vice versa. And in the centre of consciousness in which he sees the left half of his visual field, and is aware of what he is doing with his left hand, this person is quite unaware of what he is doing with his right hand, and vice versa.

In seeing what is in the left half of his visual field, such a person is quite unaware of what he is now seeing in the right half of his visual field, and vice versa. And in the centre of consciousness in which he sees the left half of his visual field, and is aware of what he is doing with his left hand, this person is quite unaware of what he is doing with his right hand, and vice versa. One of the complications in the actual cases is that for most people, in at least the first few weeks after the operation, speech is entirely controlled by the right-handed hemisphere.

Key Concepts

• SOME recent medical cases provide striking evidence in favour of the Reductionist View.
• The effect, in the words of one surgeon, was the creation of ‘two separate spheres of consciousness’.
• psychologists can thus present to this person two different written questions in the two halves of his visual field, and can receive two different answers written by this person’s two hands.
• With both hands the person writes, ‘Only one’.
• With one of his hands the person writes ‘Red’, with the other he writes ‘Blue’.
• In seeing what is in the left half of his visual field, such a person is quite unaware of what he is now seeing in the right half of his visual field, and vice versa.
• in the centre of consciousness in which he sees the left half of his visual field, and is aware of what he is doing with his left hand, this person is quite unaware of what he is doing with his right hand, and vice versa.

Context

Opening of Section 87 ('DIVIDED MINDS'), where Parfit introduces real split‑brain surgery cases and associated experiments to argue that a single human being can host two separate, simultaneously unified streams of consciousness, bolstering his Reductionist account of personal identity.