Coco the gorilla might have been but turned out to be far removed (largely because she faked knowing sign language and her handlers didn’t know sign well enough to see she was faking it.)
Language comprehension is often mistaken for sentience, but everything communicates on some level. Developing a means for communication doesn’t necessarily imply sentience. By default, Mammals are conscious because of the way they propagate themselves through different means in complex environments. OTOH, bacteria are not conscious because their propagation is mostly driven by chemical responses. For example, indv. bacteria are not engaging in game theoretic interactions with other bacteria over resources for self-propagation.
Heck, we can’t be certain the person who wakes up every morning is the same as the person who went to sleep.
Yes, we can on many levels. I am not sure who says these things, any links?