peye

Paraconsistent Animals

When the facts disagree, say so, instead of pretending they don’t.

paraconsistent-animals.py · output · proof · check · try it in the playground

The question

Birds fly. Penguins don’t. Tweety is a bird and a penguin. Does Tweety fly?

Real data is often like this: two rules, or a rule and an observation, point in opposite directions. Ordinary logic breaks down here: from a contradiction it lets you conclude anything at all.

Paraconsistent reasoning is reasoning that keeps working when the data contains contradictions. It keeps the conflict contained and visible.

What we tell peye: animals and rules

fact(bird('tweety'))
fact(penguin('tweety'))
fact(bird('falco'))
fact(penguin('opus'))
fact(mammal('batsy'))
fact(bat('batsy'))
fact(fish('nemo'))
fact(bird('mythic'))
fact(observed('mythic', 'flies', 'false'))
implies(bird(X), flies(X, 'true'))
implies(bird(X), wings(X, 'true'))
implies(penguin(X), flies(X, 'false'))
implies(mammal(X), wings(X, 'false'))
implies(bat(X), flies(X, 'true') & wings(X, 'true'))
# …

Conclusions are written as flies(X, 'true') or flies(X, 'false'), so both can be recorded side by side without the program falling over.

What we tell peye: summaries

Each property gets a local summary: true only, false only, or both.

implies(flies(X, 'true') & flies(X, 'false'), flight_status(X, 'both'))
implies(flies(X, 'true') & ~flies(X, 'false'), flight_status(X, 'true_only'))
# …
implies(flight_status(X, 'true_only'), flies_safely(X, 'true'))
implies(flight_status(X, 'both'), flies_safely(X, 'undecided'))

~ means “not found”: nothing peye knows says otherwise. Decisions read only the summary, so a conflict leads to undecided, never to both answers.

What peye concludes

50 conclusions in all. The interesting ones:

inconsistent('tweety', 'flies')
needs_review('tweety', 'flies')
inconsistent('mythic', 'flies')
inconsistent('batsy', 'wings')
flies_safely('tweety', 'undecided')
flies_safely('falco', 'true')
flies_safely('opus', 'false')
moves_by('opus', 'walking')

Tweety (bird and penguin) and Mythic (a bird seen not flying) are flagged for review. Batsy the bat has wings by one rule and none by the mammal rule. Falco, Opus and Nemo are clear-cut.

Why: the proof in plain words

For Tweety:

  1. Tweety flies — birds fly, because bird('tweety').
  2. Tweety does not fly — penguins don’t, because penguin('tweety').
  3. So Tweety’s flight status is both, and Tweety needs review.

For Falco:

  1. Falco flies — birds fly.
  2. Nothing says Falco does not fly (a search that found nothing).
  3. So Falco’s status is true only, and Falco flies safely.

Checked, not just claimed

59 steps were verified against the program. Verdict: checked_with_obligations.

The 7 obligations are all of the kind called absent, one for each “not found” the decisions rely on, such as:

obligation('absent', 'theory_scoped', ~flies('falco', 'false'))

peye concluded that nothing says Falco cannot fly because it searched everything it knows and found nothing. The checker records each such absence rather than proving it, and confirmed that no step in the proof contradicts any of them.

Try it

python -m peye examples/paraconsistent-animals.py            # the answers
python -m peye --proof examples/paraconsistent-animals.py    # answers with their proof

Or open it in the playground. Delete the line fact(observed('mythic', 'flies', 'false')) and run again: the conflict disappears, and you get flies_safely('mythic', 'true') and moves_by('mythic', 'flying').

Takeaway

Contradictions in data are not a reason to give up or to guess. Kept local and labelled, they become a to-do list for a human, while everything that is clear-cut still gets decided.

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