Topos-TheoreticProbability
Descent as Epistemic Coherence
Treat uncertainty not as random noise on one big global picture, but as local beliefs that must agree where they overlap.
§0Abstract
Treat uncertainty not as random noise on one big global picture, but as local beliefs that must agree where they overlap; coherence across overlaps is enforced by descent (gluing), and Bayesian updating becomes “make the locals compatible, then glue.”
§1Minimal Background
Sheaf
A way to store data 'locally' on pieces of a space (nodes, regions, time windows) plus restriction maps that say how data looks on overlaps.
Topos
A universe where logic/probability can live. Working in a topos lets 'probability objects' behave functorially (pushforward/pullback) with crisp rules.
Descent
If local pieces agree on overlaps, there exists a unique compatible global section. Epistemically: if agents/sensors agree where they both see, they can be fused globally without contradictions.
§2Why This Reframes Fusion
Classic Fusion
Sheaf Fusion
§3A Practical Recipe
- Base space: Choose your cover (e.g., rooms in a house × time windows; or sensor fields of view).
- Stalks (local data): For each open set, store a belief object (distribution, credal set, Dirichlet parameters, subjective-logic opinion, etc.).
- Restrictions: Maps that push beliefs to smaller overlaps (marginalize variables, project coordinates, or calibrate units).
- Compatibility loss: Measure mismatch on overlaps (e.g., KL(P|Q)+KL(Q|P), Wasserstein, or an interval/credal distance).
- Inference = sheaf gluing: Minimize total incompatibility subject to each node respecting its likelihood/prior. This is the sheaf Laplacian or message-passing analogue; the optimizer returns a glued belief.
- Bayesian updates: New observation = update the local node, then re-solve the coherence problem (fast incremental solvers are possible).
- Diagnostics: Large overlap residuals pinpoint bad sensors, drift, or miscalibration; cohomology classes (if you go that far) flag "global contradictions" that no local tweak can fix.
§3.1Concrete Choices That Work
Belief Types
Overlap Loss Functions
- Gaussian: Symmetric KL or 2-Wasserstein
- Discrete: Jensen-Shannon
- Subjective logic: Transform to equivalent Dirichlet, then JS/KL
Solver
§4Worked Sketch: Gaussian Case
Setup
- Nodes: (Uᵢ) (e.g., 'Kitchen @ minute k'), each with Pᵢ = N(μᵢ, Σᵢ)
- Overlap Uᵢ ∩ Uⱼ: Both restrict to common variables via linear maps (Rᵢ, Rⱼ)
ℓᵢⱼ = W₂²(Rᵢ#Pᵢ, Rⱼ#Pⱼ)
min{Pᵢ} Σᵢ KL(Pᵢ || P̃ᵢ) + λ Σ₍ᵢ,ⱼ₎ ℓᵢⱼ
where P̃ᵢ is the sensor-updated local posterior. The solution gives glued Pᵢ*; take any global section if needed.
§4.1Connection to Epistemic Transport
- Transport: Move beliefs between contexts via restriction/pushforward; cost = your transport metric (Wasserstein/KL).
- Epistemic: Beliefs may be second-order (uncertainty about parameters); encode as richer stalks (credal sets/intervals, or hyperpriors).
- Objective: Literally optimal transport across overlaps + data terms; descent enforces that the transported beliefs agree.
§4.2Implementation Notes (Python)
# Represent the cover and restrictions (graph or poset)
cover = build_cover(rooms, time_windows)
# Pick a belief carrier
# NumPy/SciPy for Gaussians; Dirichlet for discrete
beliefs = {node: GaussianBelief(mu, cov) for node in cover}
# Build overlap list with restriction matrices
overlaps = compute_overlaps(cover)
restrictions = {(i,j): linear_restriction_map(i, j)
for (i,j) in overlaps}
# Solver: ADMM over overlaps
def sheaf_glue(beliefs, overlaps, restrictions, lambda_reg):
while not converged:
# Push to overlaps
for (i, j) in overlaps:
Ri, Rj = restrictions[(i,j)]
mismatch = compute_wasserstein(Ri @ beliefs[i],
Rj @ beliefs[j])
# Take proximal/gradient steps
beliefs = update_beliefs(beliefs, mismatch, lambda_reg)
return beliefs§5Where This Shines
Heterogeneous Sensors
Different units, frames, modalities → encode conversions in restriction maps. The sheaf structure naturally handles the translation.
Partial Observability
Overlaps are small but sufficient to glue. Even with limited shared context, coherent global beliefs emerge.
Drift Detection
Rising cohomology/residuals signal structural inconsistency. The framework provides built-in diagnostics for "something is wrong."
§6Quick Starter Roadmap
Gaussian Sheaf Fusion
Room-by-room temp/pressure network; symmetric KL on overlaps; ADMM solver.
Subjective Logic Bridge
Subjective-logic to Dirichlet bridge; JS overlap; outlier-robust penalties.
Wasserstein Transport
Wasserstein-regularized transport + topology-aware diagnostics (flag loops with persistent residuals).
Streaming Updates
Streaming updates with incremental gluing; per-edge health scores for monitoring.