What differentiates humans from machines? We feel. We want to feel pleasure and to abstain from pain. Now, I can build (with a little help of my friends) a machine that will alert/make sound whenever it feels a high pressure on it, but you wouldn't mind to hit it as you know that it doesn't feel pain.
But what are these feelings (like the pain you feel and can't stand)?
Could machines be able to feel (specifically pain)? How?
We were naturally designed to respond to injuries with unpleasant feelings of discomfort, sorrow, and pain -- for the purpose of extending our lives as long as possible. Because if we avoid causing such negative feelings in ourselves and others, we tend to survive longer.
So it is a defensive survival mechanism.
Therefore, if we enjoy life and desire to live and grow -- any injuries we receive that inhibit our goal of life and growth will cause us pain and/or discomfort, because we will naturally maintain a negative attitude toward injuries. A normally rational negative attitude toward pain is generally healthy, because it tends to promote health, life, and growth.
Conversely, if we actually hate life and growth -- we usually somehow take pleasure in our own or others' pain, or we are generally unaffected by it. Such cases fall somewhere in the realm of sado-masochism, and are unhealthy (aka deviant) because they do not promote health, life, and growth.
(But because they are frequently met with societal disapproval, such expressions of hatred toward healthy life are usually sublimated in various ways in attempts to present a more acceptable self image or persona.)
From SEP:
No rational person seeks an injury for itself, and every rational person avoids injury for itself. An injury is something that we should always want to avoid because it is a physical change which impairs the function of a particular body part, and we all have reason to want our bodies to function properly (VV 116–117; VV 122). General facts concerning human beings support the idea that it is necessary for rational people to want their limbs intact...
...[Philippa Foot] rejects the narrowly hedonistic view that the value of life is determined by the balance of pleasure versus pain it promises. On her view, life is often still a good to someone who is suffering and who is likely to continue in such a state. Yet she also argues that merely being alive without suffering is not a good (VV 42). What is of value, on her view, is the ordinary human life that contains at least a minimum of “basic human goods”, which include: that a man is not driven to work far beyond his capacity; that he has the support of a family or community; that he can more or less satisfy his hunger; that he has hopes for the future; that he can lie down to rest at night. (Ibid.)
The answer is No. Because machines are not organically, biologically alive, they are not invested in the debate (outlined above) over whether or not life has value.
And from Psychology Today:
...Integrated Information Theory...makes very clear that a perfectly accurate computer simulation of a brain would not have consciousness like a real brain...Neuroscientists Giulio Tononi and Christof Koch, who established the theory, do not mince words on the subject:
IIT [Integrated Information Theory] implies that digital computers, even if their behaviour were to be functionally equivalent to ours, and even if they were to run faithful simulations of the human brain, would experience next to nothing.
So it is safe to say that without consciousness, no machine is capable of functioning like a real, complete, healthy human being.
The definition that you gave is not very good. As you noticed yourself, it is possible for machines to have sensory input for high pressure (or heat, cold, damage of limbs, etc ...), just like humans do. It is also possible for machines to enter certain states that interrupt their usual activities, just like humans who feel pain enter a certain state of mind that interferes with their attempts to discuss philosophy (among other things).
That does not mean that machines have consciousness, and that they are not philosophical zombies. The debate of what makes consciousness, whether it is possible to achieve consciousness and sentience with some complex (self-learning) algorithm, or whether it is something beyond our understanding, something undefinable ("divine spark"), is far from conclusive.
Q1: What differentiates humans from machines?
Answer: Many things differentiate humans from machines, but perception-wise, what differentiates humans from machines is conscious perceptions: humans have conscious perceptions but machines do not. Conscious perceptions are perceptions that we can be conscious of. This means that we know what it is like to have those perceptions, such as we know what it is like to see the color red, to hear a musical sound “C”, to smell the rose odor, to taste the sweet honey, and to feel the pain when we have the perceptions of the color red, the musical sound “C”, the rose odor, the sweet honey, and the pain, respectively. (ref 1, ref 2, ref 3, ref 4, ref 5)
This is in contrast to unconscious perceptions, which are perceptions that occur unconsciously and that we cannot be conscious of. That is, we do not know what it is like to have those perceptions, such as we do not know what it is like to have blood sodium level at various levels, to have blood sugar at various levels, and to have various blood hormones at various levels, although we are unconsciously aware of them and react to them all the time.
Machines’ perceptions are like our “unconscious perceptions”. Machines just acknowledge various signals and react to them (such as acknowledge the high pressure on them and react to them, in your example) without knowing what it is like to have those signals – the knowing of what it is like to have those signals does not occur in machines. This is similar to what happens when we have unconscious perceptions, in the preceding paragraph.
Now how do we know that a machine does not know what it is like to feel signals? The answer is to know what it is like to feel signals is an additional phenomenon to the phenomenon of feeling the signals. The additional phenomenon of knowing what it is like to feel signals needs an additional circuit. Present-day machines are built with circuits to just acknowledge various signals, so they just acknowledge signals as they are built to do. They do not and cannot do something more or less than what they are built to do. (This can be checked by checking all bits of data in their systems whether they represent something else that they are not pre-designed to represent. And so far, it has never been found that there are unexpected bits of data in normally-functioning computer systems.)
To recap, to know what it is like to acknowledge signals needs an additional circuit to create the additional phenomenon of knowing what it is like to acknowledge signals. But, nowadays, there are no such circuits because we don’t know how to build such circuits yet.
Q2: But what are these feelings (like the pain you feel and can't stand)?
These feelings are our conscious perceptions, as described above. Some philosophical literature calls them qualia. (See ref 1 - 5 above.)
Q3: Could machines be able to feel (specifically pain)? How?
Machines can feel (e.g. pain) as we do if they can have conscious perceptions as we have. But to have conscious perceptions like us, they must have circuits that can create conscious perceptions (in addition to circuits that just create unconscious perceptions, or acknowledgment, of the signals). The problem is, at present, we do not know how to build such circuits yet. (see section 6.6.2 this chapter) And it is still controversial whether, theoretically, such circuits are possible in machines.
