mind in the source would then be not a creation de novo but a development of mind
from unrecognizable into recognizable." (Sherrington, 1942)
Indeed, given our current, limited knowledge of the necessary and sufficient
conditions for consciousness in humans, we cannot, as yet, rule out even more remote
possibilities. If the ability to represent and respond to the world, or the ability to
modify behaviour consequent on interactions with the world are the criteria for
consciousness then it may be that consciousness extends not just to simple
invertebrates (such as Planaria) but also to unicellular organisms, fungi and plants.
For example, the leaflets of the Mimosa plant habituate to repeated stimulation, i.e.
the leaflets rapidly close when first touched, but after repeated stimulation they re-
open fully and do not close again while the stimulus remains the same. Surprisingly,
this habituation is stimulus-specific. For example, Holmes & Yost (1966) induced
leaflet closure using either water droplets or brush strokes, and after repeated
stimulation (with either stimulus) habituation occurred. But, if the stimulus was
changed (from water drops to brush strokes or vice-versa) leaflet closure re-occurred.
For many who have thought about this matter, the transition from rudimentary
consciousness in animal life to sentience in plants is one transition too far. Perhaps it
is. It is important to note however that a criterion of consciousness based on the
ability to respond to the world does not prevent it. Nor, on this criterion, can we rule
out the possibility of consciousness in systems made of materials other than the
carbon-based compounds that (on this planet) form the basis for organic life. Silicon-
based computers can in principle carry out many functions that, in humans, we take to
be evidence of conscious minds. So how can we be certain that they are not
conscious?
One should recognise too, that even a criterion for the existence of consciousness
based on the ability to respond or adapt to the world is entirely arbitrary. It might for
example be like something to be something irrespective of whether one does
anything! Panpsychists such as Whitehead (1929) have suggested that there is no
arbitrary line in the descent from macroscopic to microscopic matter at which
consciousness suddenly appears out of nothing. Rather, elementary forms of matter
may be associated with elementary forms of experience. And if they encode
information they may be associated with rudimentary forms of mind.
Does matter matter?
Many would regard Whitehead’s views as extreme (I give my own assessment
below). But there is one position that is even more extreme - the view that the nature
of matter doesn’t matter to consciousness at all. At first glance, it might seem
preposterous to claim that matter doesn’t matter for consciousness. But, surprising as
it might seem, it is a logical consequence of computational functionalism—one of the
most widely adopted, current theories of mind. As John Searle has noted, it is
important to distinguish this position from the view that silicon robots might be
conscious. For him, human consciousness in spite of its subjectivity, intentionality, and
qualia is an emergent physical property of the brain. If so, a silicon robot might have
consciousness. But this would depend not on its programming, but on whether silicon
just happens to have the same causal powers (to produce consciousness) as the carbon-
based material of brains.