During my first term of full-time teaching in 1977, I used astrology as a case study in my course in the philosophy of science. The goal was to guide the students through the differences between sciences and pseudosciences, which are fields that claim to be scientific but aren’t. It was easy to show that standard ways of demarcating science such as falsifiability and verifiability don’t work, but I was embarrassed to approach the end of the term without providing the students with an alternative. 

So after the term was over, I came up with my own definition:

A theory or discipline that purports to be scientific is pseudoscientific if and only if:

1.  It has been less progressive than alternative theories over a long period of time, and faces many unsolved problems; but

2.  The community of practitioners makes little attempt to develop the theory towards solutions of the problems, shows no concern for attempts to evaluate the theory in relation to others, and is selective in considering confirmations and disconfirmations

This definition was published in my 1978 article, “Why Astrology is a Pseudoscience”, which has been frequently reprinted in textbooks. 

I later realized, however, that it was more informative to provide a list of typical features of pseudoscience rather than a strict definition, and hence provided profiles of science and pseudoscience in my 1988 book, Computational Philosophy of Science.  Most recently, I was asked to write an introduction to the philosophy of science for a science education audience, so in this article updated the profiles to include the following differences:

1.  Science explains using mechanisms, whereas pseudoscience lacks mechanistic explanations. 

2.   Science uses correlation thinking, which applies statistical methods to find patterns in nature, whereas pseudoscience uses dogmatic assertions, or resemblance thinking, which infers that things are causally related merely because they are similar.

3. Practitioners of science care about evaluating theories in relation to alternative ones, whereas practitioners of pseudoscience are oblivious to alternative theories.

4. Science uses simple theories that have broad explanatory power, whereas pseudoscience uses theories that require many extra hypotheses for particular explanations.

5. Science progresses over time by developing new theories that explain newly discovered facts, whereas pseudoscience is stagnant in doctrine and applications.  

The major addition over my earlier accounts is the first claim that science uses mechanisms whereas pseudoscience ignores them.  A mechanism is a system of parts whose interactions produce regular changes, as in a bicycle which has pedals, a chain, and wheels that move it forward.  Sciences like biology have an abundance of mechanisms, for example the operations of cells that produce life.   In contrast, pseudosciences like astrology are mechanism-free, as no one has a clue how the configuration of the stars at a person’s birth could affect personality.   Moreover, astrology is not based on statistical correlations, but rather on vague notions of resemblance such as that the red planet Mars is associated with being warlike. Astrologers don’t evaluate their theories in comparison to alternative theories of personality such as genetics and social learning

So why does astrology remain popular? I think the best answer is motivated inference: astrologists generally tell people things that they want to hear. I pointed out in my 1978 article that astrology would have fewer enthusiasts if horoscopes tended less toward compliments and pleasant predictions and more toward the kind of analysis that Mother Jones magazine applied to people born under the sign of VIRGO:  You are the logical type and hate disorder. This nit-picking is sickening to your friends. You are cold and unemotional and sometimes fall asleep while making love.  Virgos make good bus drivers. 

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