Pain.

Chronic pain is a form of madness which one has no more chance of escaping from than an asylum...

Generally, we accept such pain as we do because we, as Kirkagard pointed out, suffer as guilty... We do not have to know much about human behavior or our own to know we deserve all the pain we get..

Animals, seeming to be generally unconscious cannot judge their pain as we do, as excessive, or deserved, or fair, or unjust.. We do not only judge our pain, but judge how well we deserve pain...

Concepts, and forms, are judgements, and as Kant said, or suggested, knowledge is judgement... Well, we conceive of everything, even our mental and moral states... All our thoughts are carried and communicated by forms and concepts, and it is not just pain or pleasure we judge, but every single thing we can conceive of...Pain is real whether we can conceive of it exactly or not... Reality would still be real even if we could not conceive of it by way of forms and ideas, but without defining bits of reality with forms until we have built up a total picture out of pieces, we would not be able to grasp the whole as well as we do, one piece at a time...

I have not read the article, but Maslin may just be saying that only language users only can be aware that it is pain that they are experiencing. That is, that the name of their sensation is "pain", and not that animals cannot experience pain (as you seem to think he is saying). And, of course, that is true. Only if the organism who feels the pain has the word "pain" can he be aware that it is pain he feels. So I think that you may be misreading Maslin. He is not saying that animals don't have pain, but that they are not aware that it is pain. And I think that is true.

Whether our knowledge of our own mental states is infallible is something that is controversial. But I have sometimes not been sure whether my sensation is pain, or something different like a mild discomfort. But that is a different question from the one you raised.