Well, under terrestrial Earthlike conditions, anyway.

In the deep ocean, the evolutionary pressure towards sight is so slight that most animals gradually lose it (there is too little light).

Is that really true? There are many photo luminescent creatures there, so light must be important to them. I'm thinking they appear to be blind to us in normal daylight, perhaps because their sensitivity is to extremely low levels of light. In their environment, they can see perfectly well?

Deep ocean may be an unusual case because species and individuals can relatively easily move between different layers. Unlike caves.

But marine creatures have many different adaptations. Cetaceans IIRC have relatively good visual acuity, but are monochromants. Lots of animals lose color vision. Some animals develop huge eyes to pick up tiny bits of light; some have a bioluminescent lure for those fish with sight. Others go blind. But no marine creatures adapted to deep ocean are going to have the level of visual acuity and sensory dependence on sight that terrestrial creatures often have.

Blindness is a continuum.

true, but consider other sighted animals but which have a different dominant sense: dogs (smell), moles (touch), platypus (electric-fields)

even sight-dominant animals may have radically different vision to ours. Cats for example are more sensitive to motion than to static resolution

How we obtain our data about the world is a huge influence over how we conceive of it. I wonder how different programming would be were we to do it entirely by touch

All of your examples are related to food acquisition. Consider /imagine the non-sighted analogues of them, and how 'successful' they are/would be e.g. a worm the size of a mole[0] & how trivial it would be for predators to prey on it.

[0]: not these ones tho https://en.wikipedia.org/wiki/Giant_Gippsland_earthworm

yes i guess the evolution of our vision was driven in part by hunting (being able to resolve animals at a distance) and gathering (being able to discern colours of berries)

My point is: these creatures evolved more specialised senses to improve their food-gathering, but they also all have eyes. Perhaps eyes convey a general advantage, regardless of how you find food?

also the medium in which we primarily exist (i.e air). If we existed in water, or in soil, then the collection and classification of light would not be as important.

I think the vast amount of seeing marine creatures is a massive counterexample. Evolutionary biologists think that the ability to detect any photons grants an advantage: in environments where there is light, eyes evolve, even when the light is dim. It's only in completely lightless environments that eyes no longer provide an advantage (i.e. abyssal plain or caves).

Agreed, the sea creatures have evolved to capture even more light. I'm only guessing, but I would think the pressure and temperature sensors on fish would be more important than sight?

You mean, sensors to detect at a distance? For temperature, most deep-sea creatures have large amounts of insulation to survive the cold & thus would be difficult to detect (especially since water is also an insulator). For pressure, those exist and are called tendrils/antennae.

It is theorized that cephalopods can see color through chromatic aberration because of the unusual shape of their pupils. They don't have cone cells like humans.

http://news.berkeley.edu/2016/07/05/weird-pupils-let-octopus...

Related: https://qz.com/192874/is-the-oculus-rift-designed-to-be-sexi...

>essentially guaranteed any sentient biological

The evolutionary pressure toward radar or at least sonar is pretty fucking big too (huge benefit) so where's mine?

You have eyes + aren't active while finding food/avoiding predators in an environment with limited visibility & great sound conduction.

alright, what about nocturnal animals with huge sensitive eyes ? or why not evolve searchlights or even lasers ?

If you are a predator and carry a search light, chances are your prey will learn what your search light means. If you are prey, would you think it is a good idea to advertise "there's something to eat over here"?

  > searchlights or even lasers

Think about the return on investment on these: how much energy to grow and power vs. the improved food gathering. Also: would this make the creatures easier to find by predators?

Galagos developed large eyes rather than sonar, true. I refer you to my other comment:

  > Perhaps eyes convey a general advantage, regardless of how you find food?

I think we can all agree that my point is made, and it's easy to imagine sentient extraterrestrials without photoreceptors.

Most people get a lot of information from their environment from passive sonar...how "echoey" a room is, for example. People can take this ability much further. https://www.youtube.com/watch?v=uH0aihGWB8U

I suppose sonar could be an alternative worth considering.