To be a critical thinker is to be comfortable with uncertainty and with the limits of human knowledge; to be aware of the many flaws and limitations of human intelligence, to be flexible in the face of new ideas or information, but to not be afraid to acknowledge that some ideas are objectively better than others.We acknowledge that we can't know things for sure, in the sense of having 100% certainty, but we can increase our confidence through evidence and through reasoning. So, keeping that in mind, I accept that the world is pretty much like our senses, and our science shows it to be. To anyone who would make a counter-claim, I ask that they give me good reason to accept their alternative. Barring such evidence, I see no reason to seriously entertain an alternative.
I am also a Philosophical Naturalist - I think that the observable universe is all there is, is all there is evidence of, and is all that we need to concern ourselves with. Other metaphysics (Idealism, Subjectivism, Relativism, Solipsism, Dualism, Monism, etc.) so blatantly contradict common experience or are so obscure and ad-hoc that I can't accept them. The philosophical choice which the evidence of our experience best supports is Realism. When we look both ways when we cross the street it is because at our core, we are all realists.
Epistemology
Regarding how we come to know the world and ourselves (my epistemology), everything we know comes from outside through our senses and inside from our minds (which is really just our minds sensing activity in other parts of our minds). There is no special revelation from supernatural entities, or miracles performed by them to get things done in the world. The real world exists without their help, and although we may be constrained in our ability to experience it by our limited senses and minds, it is out there, and we are in it. It is widely held that the most powerful support for Realism is the "no-miracles argument", according to which the success of science and realism would be miraculous if scientific theories were not at least approximately true descriptions of the world.
The nature of reality
To those who say we can't experience "ultimate reality", I question if this is even a meaningful concept (what is it other than the "actual reality" we find ourselves in?). If we can't experience it, what indication is there that it is even a real thing? I would assert we are living in "ultimate reality" - this is it. This is the only reality for which there is any evidence. Every one of our cells is constrained to operate in the 4 dimensional space-time web which constitutes reality. In fact, we can't escape it - where else would we go? Proponents of the idea of an unreachable "ultimate reality" have become enthralled by the invigorating possibility that something lies beyond our mundane world. But simply desiring and being able to conceive of such a thing in no way causes it to exist, any more than Anshelm's belief that there is a perfect god means that such a being really does exist. The desire to "transcend" and believe in a transcendent reality probably appeal to our human natures more than they refer to an actual "other" existence. As Karen Armstrong said,
The word, transcendence, means that which goes beyond our normal experience. Throughout human history, men and women have experienced a hidden sacred dimension of life which is beyond their normal thoughts, ideas, and experiences. We seek ecstasy. We seek to go beyond ourselves and experience this other, this dimension of "something else". And if we don't find it in religion, we look for it in other activities - in art, in sport, in sex, in drugs even (mistakenly). This is the way we are constituted. We are, as human beings, able to have experiences and to conceive of ideas that go beyond what we can grasp. This is part of the human condition.It is true that we experience the universe through the filter of our senses and what our instruments tell us, and is constrained by the structure of our mental processes. What we see and know of the universe may only be an approximation of some more "high-resolution" view of reality, but every new scientific observation brings us closer to understanding it, and allows us to see deeper and farther into it. That journey towards greater understanding has not come from religion or mythology, but from controlled observation, experiment, and theory. If there is anything else that is knowable, it will probably come about as a direct result of the continued investigations of mathematics, the sciences, engineering, exploration, and creation (both scientific and artistic). Already science has shown us a universe trillions of times larger and infinitely more intricate than anything conceived of by the religions and myths of pre-scientific societies.
Do things exist which aren't material - yes, of course. It really depends on what we mean by the word, "exist", which is probably too weak and non-specific a word to encompass what is we refer to when we say it. As Dale Horvath said in a "Walking Dead" TV episode, "Words can be meager things - sometimes they fall short." Frequently "existence" is applied both to material objects and to non-material entities: processes, flows, relationships, behaviors, evolution, and other dynamic aspects of complex (i.e., more than one object involved) systems. Obviously, emotions and feelings "exist". We humans (and probably other animals) clearly have inner lives. Love, hate, loyalty, treachery, and all the other human emotions "exist", using a loose definition of the word. They exist more in the sense of how relationships or processes exist. There are no emotional "atoms" that can be weighed and measured. But it is probable (neuroscience and evolutionary biology are very close to showing this) that the human emotions that give us reasons to keep on living are outgrowths of physical processes in the body/brain system. That doesn't make them any less important, just as knowing how a rainbow, opera, flower, or sonnet works doesn't make them any less beautiful. To those who would say, "if you can accept that love and loyalty exist though you can't hold them in your hand, then why can you not also accept god?". Unlike love and loyalty (which are internal, personal mental states), there is no evidence of a external god outside the mind. There is no reason to believe in this type of god any more than there is a reason to believe in the infinity of other concepts that can be thought of which also don't really exist. If we can limit our definition of god to a personal, internal, mental or emotional state that might be more acceptable. But that is not generally what believers in god intend when they argue for his existence. Knowing that a few immaterial "things" exist (emotion, change, the future, the past) is no argument for thinking all immaterial things exists. Each requires its own rationale and reason for deserving our belief. One cannot conclude that every immaterial thing exists because some immaterial things exist!
Our mental model of the world
Material objects (including ourselves) are in the world. Those objects interact - they have relationships to each other and influence each other. Processes and phenomena occur, objects change internally and with respect to each other through time. Because of our cognitive apparatus, we create models of both the objects of the world, their relationships, and how both of these evolve through time. We see the universe work as it does, and build intricate mathematical and logical systems that correspond to the world.
"Embodied mind" theories hold that mathematical and logical thought is a natural outgrowth of the human cognitive apparatus which finds itself in our physical universe. For example, the abstract concept of number springs from the experience of living in a world where there are discrete objects that can be counted. I think it is quite probable (though no means certain) that if the universe did not have this feature (separate, countable objects), that the theory of number would probably not have come about. Although it is not the dominant theory, I think that logical systems are a result of the human propensity to create mental models of their experiences. We construct, but do not discover, mathematics. Embodied mind theorists explain the effectiveness of mathematics by arguing that mathematics was constructed by the brain in order to be effective in this universe. It is part of our natural "model making" cognitive functioning. With this view, the physical universe can thus be seen as the ultimate foundation of mathematics: it guided the evolution of the brain and later determined which questions this brain would find worthy of investigation. It is possible that because Mathematics is an enterprise whose purpose is to describe and manipulate many types of logically consistent systems, and because our world is one such system, it should be no surprise that there are certain branches of mathematics that can describe (and predict and explain) phenomena in our world. Is our world logically consistent? Of course - if it were not, it would implode in a giant flash of improbability!
Seriously, when our theories contradict each other (such as is the case with the wave/particle theory of light or the breakdown of relativity equations inside a black hole) it is an invitation to further research, not a threat to reality. These apparent internal contradictions are more likely to indicate our inadequate or incomplete physical models rather than an actual incompatibility of reality with itself. Case in point - when James Faraday saw electric current moving a compass needle at right angles to the current, he didn't question the sanity of the universe, but concluded (correctly) that there were new laws yet to be discovered.
But we also have within mathematics many concepts that do not correspond to any real things in our world. In this view, the domain of concepts which mathematics can address includes and goes beyond the world we find ourselves in. It can describe realities which don't actually exist. There have been branches of mathematics which, for decades, were considered useless and inapplicable dead ends, only to find correlates in the external world at some future point (Boolean algebra had no immediate use until Russel used it in an attempt to find a foundation for all of mathematics, and it found even greater use later for circuit design and software development).
Emergence and the Laws of Nature
Regarding "laws of nature", I group them together with mathematical and logical objects. The "laws of nature" do not cause the universe to be as it is. The universe is already as it is, and the laws are our human attempt to organize these experiences in ways that we can describe them, explain them, and predict them. An apple doesn't fall from the tree because Newton came up with the formulas for gravitational attraction, the formulas describe (and predict) how objects fall, and how they will fall in the future.
I am not a reductionist, in the sense that I don't think all phenomena and processes can ultimately be reduced to their lowest primitive elements (physical laws). The ultimate in reductionism would be to claim that quantum theory should eventually be able to explain not only how subatomic particles behave, but how atoms and molecules form, how cells come into being, now animals work, and finally how humans and human societies work. No, I am an "emergentist". I think new properties and behaviors spontaneously emerge as complexity increases. The characteristics of water (its wetness, its freezing point, boiling point, and chemical properties) spontaneously emerge as water molecules form from hydrogen and oxygen. We cannot "average" the characteristics of two hydrogen atoms and one oxygen atom to predicts what their chemical combination (H2O) will be like. Nor can we look at any of its components (hydrogen and oxygen) to predict phase changes and it "wetness". The properties of water emerge as water (a more complex thing) is built from oxygen and hydrogen (less complex things). As molecules form, organic compounds are created, life emerges, intelligence evolves, and civilizations are born and die, new properties and behaviors spring into existence with each change in structure and complexity. The ultimate in complex properties - consciousness, intelligence, and human culture, exist in ourselves and our civilizations. Some physicists think that the lowest level of complexity - the subatomic particles themselves - may also be emergent entities that are born from the interaction of fields that pervade space (as discussed in Every Thing Must Go, by James Ladyman).