On the Learnability of Physical Concepts: Can a Neural Network Understand What's Real?

TL;DR

This paper explores the limitations and potentials of neural networks in understanding physical concepts, emphasizing the importance of architecture, data interaction, and active perception in bridging the signal-to-symbol gap.

Contribution

It provides a theoretical framework analyzing what classes of physical concepts neural networks can learn and how active perception is crucial for understanding physical entities.

Findings

Feed-forward networks cannot learn complex concepts.

Recurrent architectures can represent more concepts but may not learn them from finite data.

Active perception is essential for understanding physical objects.

Abstract

We revisit the classic signal-to-symbol barrier in light of the remarkable ability of deep neural networks to generate realistic synthetic data. DeepFakes and spoofing highlight the feebleness of the link between physical reality and its abstract representation, whether learned by a digital computer or a biological agent. Starting from a widely applicable definition of abstract concept, we show that standard feed-forward architectures cannot capture but trivial concepts, regardless of the number of weights and the amount of training data, despite being extremely effective classifiers. On the other hand, architectures that incorporate recursion can represent a significantly larger class of concepts, but may still be unable to learn them from a finite dataset. We qualitatively describe the class of concepts that can be "understood" by modern architectures trained with variants of stochastic gradient descent, using a (free energy) Lagrangian to measure information complexity. Even if a concept has been understood, however, a network has no means of communicating its understanding to an external agent, except through continuous interaction and validation. We then characterize physical objects as abstract concepts and use the previous analysis to show that physical objects can be encoded by finite architectures. However, to understand physical concepts, sensors must provide persistently exciting observations, for which the ability to control the data acquisition process is essential (active perception). The importance of control depends on the modality, benefiting visual more than acoustic or chemical perception. Finally, we conclude that binding physical entities to digital identities is possible in finite time with finite resources, solving in principle the signal-to-symbol barrier problem, but we highlight the need for continuous validation.