torchmodal 0.2.2

Differentiable Modal Logic for PyTorch

torchmodal makes the modal operators □ (necessity) and ♢ (possibility) into trainable neural layers over a Kripke structure: every truth value is an interval [L, U] that provably brackets the crisp modal-logic answer, and the accessibility relation between worlds can be learned by gradient descent instead of being specified.

📣 Oral presentation at NeSy 2026 — the 20th Conference on Neurosymbolic Learning and Reasoning. Modal Logic Neural Networks, Antonin Sulc (Lawrence Berkeley National Laboratory) and Noor Naddour (The University of Queensland). PMLR vol. 284. Read the paper →

A classic epistemic puzzle, in ten lines

Three children have muddy foreheads. Each sees the others but not themselves. Their father says "at least one of you is muddy", then asks repeatedly whether anyone knows. Nobody does — until the third round, when all three suddenly do. Nothing new is ever observed; the only information is that nobody else could answer.

import torch
from itertools import product
from torchmodal import functional as F

worlds = list(product([0, 1], repeat=3))          # 3 children, muddy or not
here = worlds.index((1, 1, 1))                    # all three really are muddy

for rnd in range(3):                              # each round rules out more worlds
    alive = torch.tensor([float(sum(w) > rnd) for w in worlds])
    # child 0 sees the others but not itself:
    A = torch.tensor([[float(w[1:] == v[1:]) for v in worlds] for w in worlds])
    muddy = torch.tensor([[float(w[0])] * 2 for w in worlds])
    L, U = F.necessity(muddy, A * alive, tau=0.05)[here]
    print(f"round {rnd + 1}: child 0 knows it is muddy -> [{L:.3f}, {U:.3f}]")

round 1: child 0 knows it is muddy -> [0.000, 0.000]
round 2: child 0 knows it is muddy -> [0.000, 0.000]
round 3: child 0 knows it is muddy -> [0.920, 1.000]

The textbook answer — no, no, yes — falls out of the modal operator alone, and every interval contains it. See examples/muddy_children.py for the full version, which checks all three children and asserts soundness at every round.

What makes this different

The combination in the first row is what is unusual: other systems either fix the relational structure and reason exactly over it, or learn structure without bracketing anything. A SemanticLoss baseline ships in this package so the comparison can be run rather than argued — see examples/baseline_comparison.py.

Soundness is a property you can check

Every operator's docstring states which crisp operator it bounds, in which direction, and what the gap is. The gap is not a hand-wave — it is an exact, computable quantity:

from torchmodal import functional as F

F.box_width_entropy(A, bounds, tau=0.1)   # τ·H(w): the exact width one □ level adds

This is conv_pool(x, -x) - smooth_min(x) identically (verified to 8.9e-16 in float64), it is bounded by τ·log n, and it tells you three things at once: how loose this □ is, how many levels you can nest before the bound floors (k* = ⌈1/(τ·H̄)⌉), and how large a contradiction can hide from L_contra without producing any gradient.

Find the silently-dead term in your neurosymbolic loss

The characteristic failure of a differentiable logic is not an exception — it is a term pinned to 0 or 1 whose gradient has vanished. It raises nothing; it just stops contributing while everything else keeps training.

from torchmodal.diagnostics import gradient_health

report = gradient_health(lambda: my_modal_term(A), {"A": A})
report["healthy"]   # False
report["issues"]    # ["term 'output.L' is dead: pinned at the floor (0.0)
                    #   with no gradient to any parameter"]

gradient_health splits [L, U] bounds into their two endpoints — the dead state of a box neuron is L = 0 with U = 1, which neither column reveals on its own — and attributes gradients per endpoint. assert_has_signal(...) is the raising variant for tests. No other neurosymbolic library ships one.

Installation

pip install torchmodal

Or from a checkout (recommended while a release is pending, since features land here first):

pip install -e .

See CHANGELOG.md for what each release contains.

Quick Start

import torch
import torchmodal
from torchmodal import nn, KripkeModel

# Create a 3-world Kripke model with learnable accessibility
model = KripkeModel(
    num_worlds=3,
    accessibility=nn.LearnableAccessibility(3, init_bias=-2.0),
    tau=0.1,
)

# Add propositions
model.add_proposition("safe", learnable=True)
model.add_proposition("online", learnable=False)

# Evaluate modal operators
A = model.get_accessibility()
box_safe = model.necessity("safe", A)       # □(safe) — necessarily safe
dia_online = model.possibility("online", A)  # ♢(online) — possibly online

# Compute contradiction loss
loss = model.contradiction_loss()

Architecture

torchmodal/
├── __init__.py          # Public API
├── functional.py        # Stateless functional operators (like torch.nn.functional)
├── nn/
│   ├── operators.py     # SmoothMin, SmoothMax, ConvPool modules
│   ├── connectives.py   # Negation, Conjunction, Disjunction, Implication
│   ├── modal.py         # Necessity (□), Possibility (♢) neurons
│   └── accessibility.py # Fixed, Learnable, Metric, Attention accessibility relations
├── kripke.py            # KripkeModel, Proposition
├── losses.py            # ContradictionLoss, ModalLoss, SparsityLoss, CrystallizationLoss, SemanticLoss
├── inference.py         # Upward-downward bound propagation
├── systems.py           # EpistemicOperator, DoxasticOperator, TemporalOperator, MultiAgentKripke
└── utils.py             # Temperature annealing, accessibility builders, decoding

Core Concepts

Differentiable Kripke Semantics

A Kripke model M = ⟨W, R, V⟩ is realized as differentiable tensors:

• W (Worlds): A finite set of possible worlds — agents, time steps, or contexts
• R (Accessibility): A relation determining which worlds can "see" each other
• V (Valuation): Truth bounds [L, U] ⊆ [0, 1] for each proposition in each world

Modal Operators

Aggregators are named smooth_min / smooth_max (not softmin / softmax) to avoid confusion with the probability-normalizing torch.softmax; the old names remain as deprecated aliases.

Top-k aggregation is a parameter of the operators — nn.Necessity(tau, top_k=k), nn.Possibility(tau, top_k=k), functional.necessity(..., top_k=k) — not of the accessibility modules. Each endpoint keeps the k extreme aggregation terms (the k smallest of (1 − A) + L for L_□, the k largest of A + U − 1 for U_♢, …) and aggregates only those, so the true min/max is always kept, the bounds stay sound, the smooth endpoints are within τ·log k of the crisp value, and nothing depends on |W|. Masking A by its k largest entries before aggregation (the top_k= of the accessibility modules up to 0.2.0) was unsound and is deprecated — see the CHANGELOG.

Accessibility Relations

# Fixed (deductive mode): enforce known rules
R = torchmodal.build_sudoku_accessibility(3)
access = nn.FixedAccessibility(R)

# Learnable direct matrix (small worlds)
access = nn.LearnableAccessibility(num_worlds=7, init_bias=-2.0)

# Metric learning (scales to 20,000+ worlds)
access = nn.MetricAccessibility(num_worlds=10000, embed_dim=64)

# Attention-based (rich per-world features, asymmetric relations)
access = nn.AttentionAccessibility(input_dim=384, num_heads=4)
A = access(features)  # features: (num_worlds, 384)

# Deliberately sparsified relation (a modelling choice: each world keeps only
# its k most accessible worlds, the rest become inaccessible). This is NOT an
# aggregation optimisation — for that use top_k= on the operators.
access = nn.MetricAccessibility(num_worlds=10000, embed_dim=64, sparsify=8)

Loss Functions

# Combined modal loss: L_total = L_task + β * L_contra
criterion = torchmodal.ModalLoss(beta=0.3)
loss = criterion(task_loss, model.all_bounds())

# Sparsity regularization on accessibility
sparse_loss = torchmodal.SparsityLoss(lambda_sparse=0.05)

# Crystallization for SAT mode (forces crisp 0/1 assignments)
crystal_loss = torchmodal.CrystallizationLoss()

# Semantic Loss baseline (Xu et al. 2018), incl. mutual-exclusion constraints
sem = torchmodal.SemanticLoss()
loss = sem.forward_mutual_exclusive(probs)  # "exactly one of k" per row

Examples

Run in Colab — no install

Notebook	What it shows
Muddy children	The classic epistemic puzzle recovered exactly, with the bounds shown to bracket the crisp answer and tighten as τ → 0
Temporal epistemic read-out	G, F and K over a spacetime frame, the 2× cost of the K∘G composite, and gradient_health locating the nesting floor
Learning the constraint graph	The hidden graph recovered from valid colourings alone — edge-recovery AUC 1.000

Scripts

All examples are self-contained scripts in examples/ and can be run directly:

python examples/muddy_children.py

Epistemic Trust Learning (CaSiNo / Diplomacy)

from torchmodal import MultiAgentKripke

# 7 agents (Diplomacy powers), 3 time steps
kripke = MultiAgentKripke(
    num_agents=7,
    num_steps=3,
    learnable_epistemic=True,
    init_bias=-2.0,
)

# Evaluate "agent knows claim is consistent over time"
K_G_claim = kripke.K_G(claim_bounds)

# Learn trust from contradiction minimization
A = kripke.get_epistemic_accessibility()

Sudoku as Constraint Satisfaction

import torchmodal
from torchmodal import KripkeModel, nn

# 81 worlds (cells), fixed Sudoku accessibility
R = torchmodal.build_sudoku_accessibility(3)
model = KripkeModel(
    num_worlds=81,
    accessibility=nn.FixedAccessibility(R),
)

# 9 propositions (digits)
for d in range(1, 10):
    model.add_proposition(f"d{d}", learnable=True)

# Train with contradiction loss + crystallization
contra_loss = torchmodal.ContradictionLoss(squared=True)
crystal_loss = torchmodal.CrystallizationLoss()

POS Tagging with Grammatical Guardrails

from torchmodal import nn, functional as F

# 3-world structure: Real, Pessimistic, Exploratory
box = nn.Necessity(tau=0.1)
access = nn.LearnableAccessibility(3)

# Enforce axiom: □¬(DET_i ∧ VERB_{i+1})
A = access()
det_bounds = ...   # from proposer network
verb_bounds = ...
conj = F.conjunction(det_bounds, verb_bounds)
neg_conj = F.negation(conj)
box_constraint = box(neg_conj, A)  # must be high (true)

Formula Graph Inference

from torchmodal import FormulaGraph, upward_downward

graph = FormulaGraph()
graph.add_atomic("p")
graph.add_atomic("q")
graph.add_conjunction("p_and_q", "p", "q")
graph.add_necessity("box_p_and_q", "p_and_q")

# Initialize bounds
bounds = {
    "p": torch.tensor([[0.8, 1.0], [0.3, 0.5], [0.9, 1.0]]),
    "q": torch.tensor([[0.7, 0.9], [0.6, 0.8], [0.4, 0.6]]),
    "p_and_q": torch.tensor([[0.0, 1.0], [0.0, 1.0], [0.0, 1.0]]),
    "box_p_and_q": torch.tensor([[0.0, 1.0], [0.0, 1.0], [0.0, 1.0]]),
}

# Run inference
A = torch.eye(3)  # reflexive accessibility
tightened = upward_downward(graph, bounds, A, tau=0.1)

The upward pass evaluates every node type. The downward pass inverts each connective on both endpoints and each modal operator on the one endpoint that factorises per world:

□ upper and ♢ lower are not inverted: they bound an aggregate without saying which neighbour realises it, so no canonical per-world constraint exists. The two passes are iterated to convergence_threshold, not run once — a downward update can stale a sibling formula that shares a leaf — and a RuntimeWarning is raised if max_iterations is exhausted first.

Two Learning Modes

Limitations

These are measured properties of the implementation, not speculation. Each has a regression test in tests/test_traps.py so it cannot silently change.

• conv_pool is not monotone. Its derivative w_k·(1 − (x_k − f)/τ) goes negative once x_k − f > τ, so raising a term that is already far above the pooled value lowers the result. Soundness is unaffected, but the tempting argument "the box neuron is monotone in A, therefore the bound is sound" is not available — the correct route is monotonicity of the hard min plus the one-sided enclosure.

• contradiction has a dead zone after a modal neuron. It is identically zero, with zero gradient, until the bound crossing exceeds the box width τ·H(w) — exactly 0.1792 for a fan-in of 6 at τ = 0.1. Do not rely on L_contra as the sole guard against a degenerate optimum; anneal τ, or pair it with gradient_health.

• Each modal level costs τ·H(w) of interval width. On a densely connected frame this is τ·log|W|, which is not negligible: with τ = 0.1 and 8 fully-connected worlds, a nest of necessities floors at depth 5 and the lower bound is then dead. Compute the budget with box_width_entropy rather than assuming it. MultiAgentKripke.K_G / K_F are two levels and consume it twice as fast.

• functional.until ignores its accessibility relation. It is correct for a total order (consecutive time steps) and only for that: until(φ, ψ, A) is bit-identical for any A, no gradient flows into the relation, and cutting an edge changes nothing. Its Łukasiewicz sweep also loses 1 − L_φ per step, flooring the lower bound over a long horizon. Use until_graph for an arbitrary or learned relation.

• until_graph(quantifier="box") is sound only on a serial frame. A dead end makes □U vacuously true, so a path that simply stops satisfies the formula. Prefer the default "diamond" (EU) unless every world is known to have a successor.

Citation

If you use torchmodal in your research, please cite:

@inproceedings{sulc2026mlnn,
  title     = {Modal Logic Neural Networks},
  author    = {Sulc, Antonin and Naddour, Noor},
  booktitle = {Proceedings of the 20th Conference on Neurosymbolic
               Learning and Reasoning (NeSy 2026)},
  series    = {Proceedings of Machine Learning Research},
  volume    = {284},
  year      = {2026},
  publisher = {PMLR},
  url       = {https://openreview.net/pdf?id=uLOdtBm0Cx},
  note      = {Oral presentation. arXiv:2512.03491}
}

The proceedings version is the one to cite; arXiv:2512.03491 (doi:10.48550/arXiv.2512.03491) remains available as a secondary identifier. GitHub's Cite this repository button reads CITATION.cff.

License

MIT

Authors

• Antonin Sulc — Lawrence Berkeley National Laboratory
• Noor Naddour — The University of Queensland

Contributions are welcome — see CONTRIBUTING.md and the good first issues.

Media

• Substack https://open.substack.com/pub/sulcantonin/p/the-architecture-of-trust-in-agents?r=2p2sn8&utm_campaign=post&utm_medium=web&showWelcomeOnShare=true