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Can Neural Networks Understand Logical Entailment?

Paper · 2018 · Richard Evans, David Saxton, David Amos et al.

Summary

The paper asks whether neural networks can decide if one propositional formula logically entails another. The task works as a clean probe of how well architectures capture purely structural properties of sequences, because variable names carry no meaning and only the connectives matter. The DeepMind authors build a new dataset of triples (A, B, label), with labels computed by the minisat solver. To remove trivial cues, they generate formulas in 4-tuples so that every formula appears in exactly one positive and one negative pair. This equalises length and operator statistics across the two classes. The test splits range from small formulas to very large ones, plus a set of textbook examples. Test and validation formulas that are α-equivalent to training formulas are filtered out. The authors compare bag-of-words baselines, convolutional networks, LSTMs and tree-structured networks. They also introduce PossibleWorldNet, a TreeNet variant inspired by model-theoretic semantics that evaluates formulas across many "possible worlds". Convolutional models show an unsuitable inductive bias, tree models beat LSTMs by exploiting syntax, and PossibleWorldNet outperforms every benchmark. The work provides a testbed for studying structural reasoning and inductive biases.

Key points

  • Dataset sizes: 100,000 train, 5,000 validation, and 5,000 each for test (easy), (hard) and (big). Test (massive) has 2,230 items and test (exam) has 100 items drawn from logic textbooks.
  • Variables and difficulty: formulas use up to 26 variables (a–z). Mean 2^#vars, a proxy for difficulty, grows from about 52 in train to about 848,570 in test (massive).
  • Debiasing: naive sampling leaks shortcuts. For example, positive premises are satisfied by about 3.7 truth assignments on average versus 10.3 for negatives. The 4-tuple scheme brings length and operator-count differences to zero, leaving only a small gap in new variables (1.25 vs 1.60).
  • Anti-memorisation and augmentation: de-Bruijn normal forms are used to drop α-equivalent duplicates. During training, variables are randomly permuted within each example, similar to image crops or flips.
  • Models compared: Linear BoW, MLP BoW, CNNs, LSTMs, TreeNets and the new PossibleWorldNet. Random or majority-class guessing scores 50% because the classes are balanced.

Our summary of the paper, in our own words — not its text. The work belongs to its authors; read it at the links above.

 Institutions

Also on the byline: University of Westminster.

 Fields, topics and concepts

Cite

Cite as Richard Evans, David Saxton, David Amos, Pushmeet Kohli and Edward Grefenstette. 2018. Can Neural Networks Understand Logical Entailment?. arXiv:1802.08535.

The paper itself, not this page. BibTeX from the library's record of the work. DOI 10.48550/arxiv.1802.08535 · arXiv 1802.08535

@misc{evans2018can,
  title = {{Can Neural Networks Understand Logical Entailment?}},
  author = {Richard Evans, David Saxton, David Amos, Pushmeet Kohli and Edward Grefenstette},
  year = {2018},
  doi = {10.48550/arxiv.1802.08535},
  eprint = {1802.08535},
  archivePrefix = {arXiv},
  url = {https://doi.org/10.48550/arxiv.1802.08535}
}

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