Is Psychology a Science?
The question is only as precise as the word “science”, which isn’t very precise. But I don’t propose to quibble about that word (I incline to a wide application of it); instead I will compare psychology to some established sciences and note various gaps in what psychology has accomplished compared to these sciences. We might express the upshot of these reflections as denying that psychology is a mature science, or that it is a real science, or that it is an explanatory science; what matters is the reality of the distinctions I identify. Psychology is not as other sciences are, dramatically so. It is signally lacking in the chief characteristics of the sciences, as they now exists. It is weak science, proto science, science in name but not in substance.
First consider the physical sciences—physics (pure and applied), cosmology, astronomy, and chemistry. What has physical science achieved? I would say that it has achieved success in three (interconnected) areas: origin, structure, and dynamics. To summarize: it explains the origin of the physical universe (big bang cosmology); it has uncovered the hidden composition of physical things (atoms, molecules, fields); and it has developed a dynamic theory of how the physical world evolves over time (specifying the basic forces and the laws that govern them). I shall say that it has achieved OSD success: it has established theories of how matter came to exist in its present form, how it is composed, and how it changes its properties with time. This is what we would expect of an adequate empirical scientific theory of some aspect of reality: an account of its origins, its underlying structure, and its behavior over time. Not to have answered these questions would make physics into a merely embryonic science, hardly worthy of the name (think physics in the age of Aristotle or earlier). Now turn to biology—anatomy, physiology, evolution, and genetics. It too can boast real achievements in the three areas identified: how life originated, the structure of living things, change of biological forms over time. We now know that life on earth began with bacteria some four billion years ago (though we don’t have a clear idea of how bacteria came to exist), and it has been evolving by means of mutation and natural selection ever since. We also know about the fine cellular structure of organisms, as well as the molecular structure of the genetic material. And we have a well-established theory of how organisms change over time (the aforementioned evolution by natural selection), as well as how individual organisms function biologically (blood flow, enzymes, digestion, photosynthesis, etc.) Granted, we don’t know everything about life–as we don’t know what preceded the big bang or how to integrate quantum physics and gravity—but we have made serious progress in understanding these three aspects of the biological world. Biology is well advanced in OSD studies. It is not that a student of the subject would have fundamental questions in these three areas about which biology has established nothing. What we expect of a reputable science is that it can tell us where its proprietary entities came from, how they are internally structured down to the microscopic level, and what explains change in them over time. Biology and physics satisfy these criteria.
But what about psychology–can it boast comparable achievements? The short answer is no. What theory in psychology plays the role of big bang cosmology in physics and Darwinian evolution in biology? None: psychology has no theory of how minds as they currently exist came to be. The best it can do is piggyback on biology, but there is no explanatory theory of how minds with their characteristic properties came to be—subjectivity, consciousness, intentionality, reason, introspection, and more. How did these develop from more primitive traits? How did the whole process begin? If a mind is like a galaxy, how did the mental galaxy form? Psychology just accepts minds as they are, animal and human, but it doesn’t explain how they came to be, what triggered them, what shaped them. There is no origin story in psychology. What about structure? We can say what the parts of the mind are—the analogues of bodily organs—but we have nothing to say about the ultimate constituents of the mind, especially its hidden structure. People mumble about “bits” of information, as if these were the atoms of mentation, but really this is hand waving, not solid science. There are no microscopes of the mind, no diffraction chambers, no spectral analyses, no supercolliders. Psychology makes do with commonsense divisions into belief and desire, memory and perception, emotions and sensations; but there is no elaborated theory of fundamental constituents analogous to atoms and molecules, cells and DNA. We don’t know how our mental life is built up. And what about dynamics? How does psychology explain the flow of conscious thoughts or the changing behavior of the organism? What laws are cited to predict how one thought will follow another, or how emotions influence overall mental state, or how a subject will act in a novel situation? Psychologists like to talk about various “effects” (e.g. the Zeigarnik effect), but where is the analogue to Newton’s three laws of motion? We just don’t have a theory of how a psychological system changes over time; at best we have rough hints about what might lead to what (as in “laws of association”). Where is the unified theory of psychological dynamics? Where are the equations of thought and action? A physicist or biologist encountering psychology for the first time might wonder how the subject accounts for origin, structure, and dynamic change—the basic facts she is familiar with in her own discipline—but her psychology professor will have little to say about these questions. He will report some experiments, maybe some established “effects”, but he won’t have comprehensive theories to offer in these three areas. He won’t say, “I’m glad you asked that question because we have great theories of how minds originated, what composes them, and how they change with time”. If he is honest, he will mutter in a low voice, “Good question, we’re working in it”, perhaps followed by some boilerplate about psychology being a young science and all that (but is it really any younger than physics and biology?).
Compare linguistics. Chomsky has long pointed out areas of ignorance in that field, mainly relating to the evolutionary origins of language and in the free use of language in speech (“performance”). The evolution of language is largely a mystery, especially the origins of the lexicon, and the stimulus-freedom of speech makes language use hard to subsume under predictive laws. Some progress has been made with linguistic structure, but even here it is reasonable to wonder whether we have reached linguistic bedrock. So linguistics has not achieved what the established sciences have. Linguistics is really a branch of psychology, and it looks as if psychology in general has the limitations Chomsky finds in this branch of it. There is some grasp of structure, basically extrapolated from commonsense psychology (including commonsense linguistics), though it has nothing like the depth we find in physics and biology. But the origins of the language faculty in evolutionary history, and how that faculty is manifested in action, are shrouded in mystery. Whether the mystery is temporary or permanent, contingent or necessary, is another question; what is clear is that psychology and linguistics do not have the kinds of explanatory success found in the established sciences. And what holds for linguistics and psychology also holds for sociology and anthropology (and maybe economics): how social structures and cultures came about is unexplained except in the most rudimentary terms, and there is no generally accepted dynamic theory of how they change over time (Hegel and Marx anyone?). Human history is not like the history of the physical universe or of the biological world. Freud made some heroic efforts to do for psychology and human history what physics and biology have achieved in their domains, but his efforts are not generally lauded. The simple fact is that the psychological “sciences” are nowhere near as advanced as the physical and biological sciences. They suffer from OSD deficiency. This is not, of course, the fault of psychologists, who are just too lazy or incompetent to bring the subject to maturity; it is inherent in the subject itself. It is very difficult to explain how minds originated, what their compositional structure is, and how they change over time.  I intend no aspersions on the field or its practitioners; I merely point out certain significant asymmetries. Presumably the mind has some sort of intelligible origin (it didn’t just spring into existence from nothing), and some sort of internal structure (the “cells of thought”), and some dynamic principles (not just stochastic chaos): but we are far from understanding what any of these might be. Nor do I see any relief on the horizon. It is pretty amazing that we have achieved the kind of insight in physics and biology that we have, and it didn’t happen overnight; there is really no guarantee that psychology will ever repeat these successes. Psychology might always remain a semi-science. 
 Note the contrast with the brain as an organ of the body. There is no more difficulty explaining its origin than other organs of the body; it is composed of cells that are composed of molecules; and its dynamic mode of operation is the nerve impulse that changes the brain’s state over time. We have a science of the brain, much as we have a science of matter and life, though of course it still has a long way to go. But that doesn’t provide us with the right level of explanation to account for the mind. Perhaps this is (partly) why people tend to favor neural reductionism: it enables psychology to mirror the theoretical successes of the other sciences.