I'll explain the question with an example. Consider Gustation: glucose tastes sweet, bitter gourd tastes bitter and salt tastes salty. Now, sweet obviously feels good but suppose there is an alien species whose biochemistry is different from us, they don't use glucose as a fuel. Now glucose maybe toxic to them so they experience a bad taste whenever they ingest glucose. In this case, the sensation is different!

Another Example would be Photoreception. How is it decided that light having wavelength of 700–635 nm would be considered red and not blue? I know that your answer would be "This is because light of this wavelength stimulates the red cone and not the blue one." Then my question would be why stimulating the red cone gives us the sensation of red colour and not of blue colour?

Similarly, Why does air of temperature higher than our body gives us a sensation of "Hot" and not of "Cold"?

I know that having different sensations for different stimuli affects our reproductive fitness. Those which can detect changes in the environment are more likely to survive than those which don't. Those which developed "sweet sensation" for glucose can gain more energy than those who don't. But it doesn't explain the relationship between the experience of a stimulus and the stimulus.

It seems like our own body has invented experience and sensation and these phenomena don't have any physical significance.

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    $\begingroup$ We call this a problem of qualia. Maybe this answer interests you: biology.stackexchange.com/questions/47828/… $\endgroup$ – user24284 May 15 '17 at 14:02
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    $\begingroup$ Also, have in mind your last parahraph: "It seems like our own body has invented experience and sensation and these phenomena don't have any physical significance." They don't! Sensations are idiosyncratic. A blue light with a given wavelength is captured by the same cones in you and me, and we both call it "blue", and we agree in every aspect of the experience... however, how do you know that the colour I feel is the same colour that you feel? $\endgroup$ – user24284 May 15 '17 at 14:29
  • $\begingroup$ "...these phenomena don't have any physical significance." That's a bit confusing, if you consider survival a "physical significance". $\endgroup$ – anongoodnurse May 15 '17 at 18:05
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    $\begingroup$ @anongoodnurse What I mean by "Physical significance" is that they are just illusion and don't actually exist. How anything "looks like", "smells like" or "tastes like" are just things completely made up concepts by the brain. $\endgroup$ – Shivanshu Siyanwal May 16 '17 at 10:16
  • $\begingroup$ @ShivanshuSiyanwal Don't get me wrong: your sensations do exist. However, they do not correspond to any physical property: The feeling that you have when you see a blue object is very real. However, the blueness of the object has no relationship (regarding your sensation) with the wavelength of the blue light it emits. $\endgroup$ – user24284 May 16 '17 at 12:47

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