There is an apparent intractable difficulty here though. One of the mainstream suppositions of scientists is that the world is causally closed, meaning that every event that ever happens has a full explanation in terms of antecedent material causes. This includes us human beings too, and so includes all the material processes occurring within our brains. But if everything people ever do and say are purely due to material causes occurring in their brains rather than being an expression of their consciousness, then it seems we cannot infer that other people are conscious!
How do those who advocate causal closure escape this absurdity? How can anyone who subscribes to causal closure believe that anyone else is conscious at all given that everyone's behaviour is just the result of material causes playing out? It seems we have no more reason to ascribe consciousness to anyone else than we have reason to ascribe consciousness to the Earth as it orbits the Sun, or a boulder as it rolls down a hill.
They allegedly escape this apparent intractable difficulty by espousing materialism. Materialists advocate something of highly questionable intelligibility. They maintain that consciousness is quite literally the very same thing as certain material processes. Examples are behaviourists who hold that consciousness is literally identical to behaviour. Or identity theorists who hold that consciousness is literally identical to brain processes. Or functionalists who hold that consciousness is literally identical to the causal role of such brain processes (there are many flavours of materialism).
In which case, given the behaviour of a person or the material processes occurring in their brains, their consciousness is logically entailed in much the same way that 2 + 2 = 4 is entailed. That we can look or examine the physical processes occurring in the brain and somehow, derive, have complete certitude, that that person is experiencing consciousness.
However, the materialist position here just seems to me to be straightforwardly incorrect. Regardless of whether we are looking at a person's behaviour or the processes occurring in their brains, we could never be absolutely certain that they are having conscious experiences. How could we? How does observing any material process allow me to be acquainted and have full knowledge of another person's consciousness? It just doesn't. Yes, the material processes might cause consciousness, might somehow elicit consciousness, but it's literally nonsensical to assert that such material processes are the very same thing as consciousness. And this shouldn't be of any surprise since material processes are cashed out exclusively by their physical properties -- mass, momentum, charge and so on. Conscious experiences, on the other hand, apparently wholly lack any such physical properties. Hence, by definition, consciousness cannot be the same as the correlated physical process and therefore there is no identity, nor any necessarily entailed connection, from one to the other.
The obvious alternative is to deny both causal closure and materialism. Instead, and as commonsense dictates, consciousness in and of itself really does play a causal role in the world. So, for example, the words and sentences you are reading now are the result of my consciousness. The material processes alone are insufficient. It is true that, unlike the materialist, we cannot have absolute certainty that others are conscious. But, via the notion that similar causes have similar effects, we can have a very high confidence.