In theory there is no difference between theory and practice. In practice there is.

This aphorism is widely circulated under the name of baseball catcher and manager Yogi Berra as one of his characteristic “Yogi-isms,” but the attribution is disputed. The line appears in print in computing and engineering circles by at least the early 1980s, often credited instead to computer scientist Jan L. A. van de Snepscheut (sometimes via colleagues’ recollections). Berra’s fame as a source of paradoxical, humorous one-liners likely helped the saying migrate to him in popular quotation culture. The quote is now commonly used in business, software development, and sports to underscore the gap between idealized plans and real-world execution.

The sentence turns on a neat paradox: “theory” treats practice as if it will behave exactly as the model predicts, so in theory there should be no gap. But once action begins, messy contingencies—human error, incomplete information, friction, time pressure, and unmodeled variables—create divergence. The humor comes from stating this obvious truth in a formally logical way, as if it were a theorem. Its significance is practical rather than anti-intellectual: it does not dismiss theory, but warns that models are simplifications and that successful work requires testing, iteration, and humility about what cannot be fully anticipated.

1) “In theory, there is no difference between theory and practice; in practice, there is.”
2) “In theory there’s no difference between theory and practice. But in practice there is.”
3) “In theory, theory and practice are the same. In practice, they’re not.”