Contributing to, but not disagreeing with** WhyNot**:
Another take on the rule-based source of personal ethics is to examine the rule as if it’s coming from within. That is, if the source of these rules isn’t coming from outside (parents, Kim Jong Il, God, Judge Judy), it’s coming from your own internal deliberations.
The “rule” in this case could be formulated as: “do not blackmail people.” You can make it* much* more complicated very quickly (which is one reason why philosophy can be so much fun), but don’t sweat they myriad ancillary issues for now. Do return to them as you chew, but it’s okay to oversimplify for a spell. For now, just take the generic definition of blackmail and the conventional take on its execution.
Why make that rule?
Perhaps because you think it’s right tha* everyone should live under the same set of rules*. (Another fun path is to get caught up in a recursive cycle of asking why why why at each step. But that’s* almost *as annoying as getting bogged down in semantics. Both are important, but both tend to derail conversation into fruitless one-upmanship. Bah.)
What if the general rule for everyone was the opposite: “it’s okay to blackmail people”?
Do you care about general, societal-level happiness? Do you think that a good ethical system would maximize happiness among members of society? Should an ethical system produce the happiest possible society? Again, forgive me for not getting bogged down in justifying every single premise for the moment and hyper-defining terms and whatnot. Just use basic understanding will do for now. In other words, just take some premises as a given. For now, just call individual, overall, and cumulative happiness “utils.” That’s an abstract shorthand for increased utility—warm fuzzies, if you will. A cupcake gives you some utils, a cupcake with frosting gives you just a bit more utils, and so on.
There are many ways to compare utils (though all ultimately fall short because person-to-person util comparison isn’t quite possible. Who is to say that I get more utils from a cupcake with ball bearings on top than you do from watching Doctor Who episodes?), but for now just make up a mental spreadsheet.
Add up the utils in a blackmail situation: You have blackmailer, blackmailee, and blackmailee-revealee (the revealee is the person to whom the blackmailer is threatening to expose the blackmailee’s secrets too). Take the time to consider all angles and conditions; blackmailers getting utils from success, blackmailee’s loss of utils from the process, the payment, etc. The effect on the revealee’s psyche. All under the various situations and possible outcomes.
In addition to aggregating that, consider the wider effects on society. As** Czarcasm** mentioned, consider certain behavioral changes. Consider the type of society and the overall util-levels that would exist if it were ethically (and legally) acceptable to blackmail people. What would a society where blackmailing was a profession be like? How would it affect overall paranoia? How would it affect overall trust and relationships? There have been many societies in the post-war era (and before and after) that had a culture of informants—would it be anything similar?
And how would a universal rule that blackmailing is okay intersect with other societal rules? (This hints at backtracking a bit to earlier premises.) If blackmailing is okay, then does that mean that it’s also morally okay to enforce agreements and transactions conducted under coercion? How does that change the overall calculation of society’s utils?
By taking the time to play around with many different possible paths, by establishing premises* and then* looping back to revisit them with challenges and new ideas and direction based on thoughts from later considerations, you can come to a much more solid understanding of yourself, your outlook, and why your sense of intuition is crafted the way it is.
Note, of course, that this isn’t* the *answer to why blackmailing is wrong. In fact, depending on how things work out and how you perceive the calculations, it may not lead to that conclusion at all! It’s also just one hastily written stream-of-consciousness hodgepodge of ideas that is just a sliver of one possible approach. Had the coin come up tails (I’m looking at you, Guildenstern), this thought experiment may have gone the non-consequentialist route.