Reasoning and perception
We need to draw distinctions. Can any moral reasoning be perceptual ('how can "moral reasoning" ...be construed as the act of perceiving?) and Is all moral reasoning perceptual ('to consider the whole of "moral reasoning" as a perception sounds like nonsense to me'). Whatever the relations between them, these questions are not the same. A 'yes' to the first does not commit us to a 'yes' to the second.
I'll fix on the first question because it is more basic. We can't say 'yes' to the second question if we answer 'no' to the first.
A widely held idea separates perception and reasoning. Reasoning and perception are taken to be radically distinct; and so no moral reasoning can be perceptual. I don't agree with this idea but it is supported by the following argument:
Inference is active, perception is passive. In inference, I set my
mind to work something out, whereas in perception, something 'just
comes to me'?I am subject to an occurrence that I do not make
happen, except in the minimal sense in which I can, e.g., choose in
which direction I look. Inference is experienced as structured, perception as simple: to perform an inference is, normally, to run
through a number of steps of reasoning, whereas perceiving some
thing is a step-less, instantaneous whole. So perception is quick
where inference is slow: perceiving something can happen instantaneously, inferring something normally takes time. (Dennett on
chess again (op. cit. p. 42): 'the scale of compression when one
adopts the intentional stance4 towards the two-dimensional
chess-playing computer galaxy is stupendous: it is the difference
between figuring out in your head what white's most likely (best)
move is [and] calculating the state of a few trillion pixels through a
few hundred thousand generations.') (Timothy Chappell, 'Moral Perception',
Philosophy, Vol. 83, No. 326 (Oct., 2008), pp. 421-437: 427-8; Dennett,
'Daniel Dennett, 'Real Patterns', Journal of Philosophy 88.1 (1991):
27-51: 34, 42.)
If we accept this line of argument then
Rejecting the separation between reasoning or inference and perception
If I can see that a glowing poker is hot, this is a perceptual judgement with inference built into it. Of course I cannot directly or immediately see the heat, or the hotness, of the poker but I have enough experience of heat and pokers to infer - to reason - that this kind of appearance in a poker is an indication that it is hot. The judgement that a glowing poker is hot is both perceptual and inferential. The inference will usually be unconscious in adult perceivers, experienced that is to say as a dispositional belief or habitual reaction.
Bringing perception into moral reasoning
I introduce David McNaughton at this point:
We might suppose that the only properties that can be observed [GT: perceived]
are the 'proper objects' of the five senses: touch, shape, and
texture; hearing, sound, and so on. If we adopt this austere
account of what can be perceived it is clear that not only moral
properties but a great many of the things we normally take our
selves to perceive will be, strictly speaking, unobservable. If,
on the other hand, we are prepared to allow that I can see that
this cliff is dangerous, that Smith is worried, or that one thing
is further away than another [GT: or that a poker is hot], then there seems no reason to be
squeamish about letting in moral observation. (D. McNaughten, Moral Vision, Oxford: Blackwell, 1988: 57.)
The nature of moral perception
So far all seems clear enough, by which I mean only that there appears to be conceptual space in which to connect reasoning and perception and to extend perception - including inferential perception - into the moral sphere. But is this conceptual possibility a real possibility? If moral perception or observation is to be a real possibility then there are, one or at least I would expect there to be, moral properties to be perceived. Are there any such properties? To suppose that there are is to subscribe to one of the many forms of moral realism.
Here we encounter all the problems that writers such as J.L. Mackie and Simon Blackburn raise against the likelihood or plausibility of the existence of moral properties. To begin, there is a supervenience problem for moral properties.
[I]f A has some naturalistic properties, and is also good, but its
goodness is a distinct further fact not following [logically] from
its naturalistic features {GT: it is in the lingo 'supervenient' on them], and if B has those features as well, then
it follows that B also is good. And this is a puzzle for the
realist, because there is no reason at all, on his theory, why this
should follow. If the goodness is, as it were, an ex gratia
payment to A, one to which A is not as a matter of logic entitled
in virtue of being as it is in all naturalistic respects, then it should
be consistent to suppose that although goodness was given to A,
it was not given to B, which merely shares the naturalistic features
that do not entail the goodness. (Simon Blackburn, Essays in Quasi-Realism, Oxford: Oxford University Press, 1993: 119.)
In other words, an action A (say) cannot have just, nothing but, the moral property
of goodness. It needs other properties: it must occur at a time, it must have effects or consequences, it must be done by an agent and so on. These we may call its naturalistic properties. On these properties its moral properties - in this case the moral property of goodness - supervene.
Moral realism generally involves that view that if two actions, A and B, are identical or relevantly similar in all their naturalistic properties, then they must be identical or relevantly similar in all their moral properties. If a moral realist commits herself to this view, on what is the 'must' based ?
More than that, morality is widely understood to be action-guiding. If, as a moral agent, someone capable of morality and concerned to act morally, I see some maltreatment of a child by an adult, or bullying between children, to take just two examples then I am motivated to act on what I have seen and, all else equal, to intervene on behalf of the victim(s). But no other properties have this motivational aspect; and Mackie calls them 'queer' for this reason. If I see two vehicles that will crash into each other unless I make a warning sign, this mere perception will not as such motivate me to do anything. We have to add the contingent desire to prevent harm. That desire cannot be delivered by mere perception or reasoning. Some other account must be offered of it, as Hume recognised (Hume, A Treatise of Human Nature, 1739-40: II.3.3 & III.1.2).
Further reading
Gilbert Harman, The Nature of Morality, New York: Oxford University Press, 1977: 9.
J.L. Mackie, Ethics: inventing right and wrong, London: Penguin, 38 et passim.