The thinking error that impairs lawyers' effectiveness
Bayes’ Theorem explains why smart lawyers aren’t persuasive—and how to fix it.
Most people think evaluation works like a light switch.
Either you’re right or you aren’t. Either the opposing counsel’s position is good or bad.
That’s not true rational thinking.
The game-changing theorem
Bayes’ Theorem—dating back to 18th century mathematician Thomas Bayes—reveals how to weigh evidence.
It’s probably the most important concept any rational person to learn, especially lawyers.
Key insight: Your beliefs shouldn’t be fixed. They should be probabilities that change with new evidence.
How this works in practice
Imagine you’re evaluating a personal injury case starting with certain assumptions (”priors”):
This type of accident usually involves driver negligence
This expert’s medical opinions are generally reliable
Witnesses with this personality tend to be credible
New evidence can challenge one (or more) of those assumptions.
Bayes’ Theorem teaches that a single contradictory piece of evidence shouldn’t flip your assessment. But as evidence accumulates, the scales tip.
The rational approach? Adjust your confidence level based on the total evidence.
Consider probabilities.
The best lawyers think in probabilities:
“Current evidence suggests a 70% chance of winning summary judgment.”
“This witness testimony shifts our settlement position from $X to $Y”
“The new case law reduces our confidence but doesn’t eliminate our argument.”
This isn’t wishy-washy thinking. It’s a rational assessment based on available information.
The AI connection
AI systems like ChatGPT work on the same Bayesian principles.
They’re probability machines, constantly updating their “beliefs” about the next words based on patterns in their training data.
Understanding Bayes’ Theorem helps you understand how AI “thinks”—and use it more effectively.
The practical advantage
Lawyers who understand Bayesian reasoning:
Make better strategic decisions under uncertainty
Adjust tactics as new evidence emerges
Avoid the sunk cost fallacy when cases weaken
Present more persuasive arguments to judges and juries
The bottom line
Being rational isn’t about being right or wrong. It’s about believing what’s reasonable based on the best evidence.
Lawyers who master this approach don’t just win more cases—they make better decisions at every stage of representation.
;-)
Ernie
P.S. Want to engage with other lawyers sharpening their analytical thinking? Check out my Inner Circle.
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