This is an ill-posed question.
“Science” is a term which has evolved over time. It was once a synonym for “knowledge” and referred to any field of formal study.
It now covers a subset of these, namely physics, chemistry, and biology… except maybe the more psychological bits of biology… maybe mathematics… and geography? Economics? Don’t even go there…
The boundaries are rather blurry.
While ‘science’ can be a convenient term, those kinds of study called ‘science’ have nothing essential that sets them apart from the rest:
There is no special ‘scientific method’ that sets ‘science’ apart. Different branches of science use different methods to uncover the truth.
Theoretical physics, organic chemistry, and evolutionary biology have nothing in common beyond the fact that they study something. And nor should they.
All attempts to prescribe (or even describe) a single ‘scientific method’, or a list of criteria for being science, have failed. Scientists don’t find them useful in practice. scientists study things and adapt their methods to the nature of the questions asked.
So “Is Psychology Science?” is not a good question. The answer depends on what you mean by the word ‘science’. That’s a semantic issue.
But facts are facts. So a better question to ask of psychology is: “is it true?” – or rather “how true is it?” Few fields are 100% true or false.
I think that most people are in fact asking about truth when they ask ‘Is It Science?’. But in that phrase, the sensible question of truth is mixed up with a vague jumble of less sensible questions such as (most often) ‘is it mathematical?’ or ‘is it systematic?’
What these questions tend to amount to is “Is it physics?’ Psychology is not physics, but neither is anything in biology, and even chemistry is on doubtful ground. Darwinism certainly doesn’t measure up to physics standards, but only creationists care about that. Those are not good standards to apply generally.
I’m not suggesting that all fields are equally valid and can be left to ‘police themselves’. Astrology, for example, is rubbish, because it consists of claims both implausible (given the rest of science) and empirically groundless.
But the rubbishness of astrology has nothing to do with whether or not it is ‘science’. Astrology is – on the surface at least – actually more ‘scientific’ than other fields, such as history. It is more systematic, and more mathematical. But history is truer.
So is astrology ‘science’? You can call it that, if you like. Is astrology true? No.
Whether psychology is true, that’s the big question – and one best broken down into chunks, because it’s a big field. I am concerned that much of psychology (and other fields) is not true. My blog is full of criticisms of particular claims in psychology.
But I’m not concerned – or interested – in whether it’s ‘science’.