I was having a conversation with an astrologer of my acquaintance when I began to formulate a hypothesis about how astrology works. That is to say, I began to sense how astrologers and their clients could come to believe firmly that the relative positions of stars and planets on particular dates can serve a predictive and explanatory function, shedding real light on character and motivation, in spite of the fact that the whole idea is (demonstrably) errant nonsense in the universe of Galileo and Hubble.
The particular astrologer in question asserted with confidence that he could know all he needed to know about any given individual based entirely on "their chart", without ever having met the individual in question. More than this, he asserted that he could always convince a skeptical client of the revealed truth about their character and motivation, overcoming their initial denial and leading them to recognize themselves in "their chart."
I put this together with things I've read and heard about astrology from various sources, including demonstrations by James Randi, along with ideas from cognitive science I have encountered by way of the literature on moral imagination. The hypothesis began to take on a more determinate shape. I don't claim this is an original hypothesis, but it has allowed me to find some clarity.
Here it is.
Human life and human experience are complex and ambiguous; our motivations are almost always mixed. As Whitman would have it, we contain multitudes. We can make sense of ourselves and the world around us only by actively framing our experience through conceptual schemata or mental models, highlighting some facets of our experience while ignoring or suppressing others.
In putting together a person's chart, an astrologer provides a very particular frame for that person's experience. Through some combination of strategic vagueness, keen observation, and persuasive power, a skilled astrologer can convince just about any prospective client that the chart really fits them, really reveals the truth about who they are.
Being human, prospective clients are often hungry for coherence; at least some of the time, most of us just don't know what to make of ourselves.
The thing is, our experience is complex enough and ambiguous enough, our motivation are mixed enough, that nearly any chart could be seen to fit nearly any person. This is a testable hypothesis, as sometimes demonstrated by Randi and others: people can "see themselves" in just about any chart. (An excellent double-blind study was published in the journal Nature in 1985; the research is also described here.)
Now, to be fair, it could be that astrology is just a harmless diversion. In the right hands, it might possibly serve as a useful tool for self-discovery or even therapy.
I have my doubts.
I suspect astrology is more pernicious, in part because it assumes the mantle of mystical insight and the presumed authority of an ancient practice. If I were on this basis to give credence to the pronouncements of an astrologer, I would be giving the astrologer real and significant power over me: the power to tell me who I am, to tell me what I really need and really want. It would short-circuit my autonomy and lock me in the prison of lowered expectations: I can always excuse any failing by appeal to the chart. (What do you expect? I'm a Libra!) In short, I would be trading some essential part of my human freedom for a false sense of certainty about my place in the universe.
I have no interest in what my own chart would show. If I am to be framed, I will frame myself.