Can you be sentient without first being concious? If not, and I'd argue not, then in speaking of sentience we are also necessarily speaking of conciousness.

Your assertion that sentience is perception of environment is a category error. You've mistaken casually identifying sentience in others with sentience itself.

I've fetched the definitions for all three terms and quoted them below.

Merriam-Webster

consciousness noun 1 a: the quality or state of being aware especially of something within oneself b: the state or fact of being conscious of an external object, state, or fact c: awareness especially: concern for some social or political cause

The organization aims to raise the political consciousness of teenagers. 2: the state of being characterized by sensation, emotion, volition, and thought : mind 3: the totality of conscious states of an individual 4 : the normal state of conscious life regained consciousness 5 : the upper level of mental life of which the person is aware as contrasted with unconscious processes

sentience noun 1: a sentient quality or state 2: feeling or sensation as distinguished from perception and thought

by extension...

sentient adjective 1: capable of sensing or feeling : conscious of or responsive to the sensations of seeing, hearing, feeling, tasting, or smelling 2: aware 3: finely sensitive in perception or feeling

sapient adjective : possessing or expressing great sagacity

Sentience is the capacity of sensing or feeling. That's an important distinction.

how is sense and reaction an indicator such a system carries within it a theory of mind?

I don't know and I'm not saying it is.

Then I fail to see the point or intention of your first comment. How are your semantics meant to further the discussion or clarify the reader's thinking?

we do not know what it is we are talking about.

Do we not? That's precisely why it's important to make that difference. Sentience is relatively easy to define and can be assigned with a lot of confidence whereas sapience isn't and can't.

If sentience here is taken literally to mean "physical sense ability" in the broadest sense then, yeah, sure. That's not helpful though. It's too reductive to be relevant to the topic at hand. Within philosophy of mind sentience is often referring to the ability or capacity for feeling (as in experience (joy, grief, etc)). It's a complex quality of some thinking minds and may well be a spectrum of ability whose boundaries are difficult to define.

[…] can sentience exist apart from perception of environment? I'd say it must.

Wait, really? Can you give several examples of this, then?

A person suffering from pseudocoma or "locked-in syndrome" is as close a real-world approximation I can think of. With that as a starting point we can arrive at my conclusion via thought experiment and create as many examples as you'd like.

Think of a brain in a vat. Maybe it's your brain. You're awake but you have no sensory input and, to your knowledge, no way of asserting your existence to the outside world.

It's a hellish thought. It demonstrates the importance of recognizing that one self's connection to the external is not equivelant with the self though.

Now digitize that mind. We now have an artificial intelligence. Disembodied. No sense organs to speak of. A thinking mind alone in a void.

Some variations on the Chinese Room demonstrate how thick a quagmire this discussion can be

For example, the chance for something to become sentient is never increased by a lack of stimuli, right?

Agreed! 😄

My point is not that an organism might find an evolutionary niche which requires no sense organs and yet develop sentience (though the universe is so vast that improbable is not impossible).

My point is that our measures and models cannot be the thing itself. Reponse to stimuli as you describe are mere measures we use to categorize minds based on our current models. Even if we believe our means are valid and useful and provide access to some level of truth: we cannot reasonably claim they allow total surety.