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On learning

\( \newcommand\Der{\mathrm{D}} \newcommand\dif{\mathrm{d}} \newcommand\Pmf{\mathrm{p}}% probability mass function \newcommand\Prm{\mathrm{P}}% probability measure \)

To learn something is to get better at it

Extrinsic/black-box/operational/test-score model

“Better” implies an ordering of goodness, and “get better” implies time, so we may model learning as an increasing test scores.

Caution: The agent maximizes the test score without necessarily understanding anything; the agent may learn the dataset bias instead of the desired feature; but here we do not discuss about making good tests.

Let \(s(t)\) be the agent’s test score at time \(t\).

The agent’s learning rate is \( \Der s \).

Too philosophical?

Learning

Learner learns Thing iff Learner causes itself to get better at Thing. A teacher may contribute to the improvement, but the learner itself causes that improvement.[fn::X causes Y iff the absence of X causes the absence of Y. On the other hand, X contributes to Y iff the existence of X changes the severity of Y.] Learning is self-improvement.[fn::”Learning is a an area of AI that focuses on processes of self-improvement.” http://users.cs.cf.ac.uk/Dave.Marshall/AI2/node131.html#SECTION000151000000000000000] “To learn Thing” is to become more intelligent in Thing; remember that intelligence is relative. For example, “they learn to cook” means that they are trying to get better in cooking, that is, how to cook more tasty food with less effort in less time. “Learning X” means finding a way to use the brain more efficiently for X so that X feels more effortless.

On the prerequisites of learning

In order for learning to be possible at all, there has to be an intersection between the learner’s architecture and the learnee’s inherent structure. We can learn some laws of Nature because there is an intersection between the way we learn/think and the way Nature works.

What is the absolute minimum requirements for learning?

Learning requires feedback and changeable internal state. How do we formalize “experience”? “Experience” can be modeled by a sequence? experience? mistakes? memory?

Philosophy of learning?

Piaget’s constructivism vs Papert’s constructionism, Edith Ackermann

Intelligence without learning

What is the relationship between intelligence and learning? Can we have one without the other? Yes. A system that stops learning after it obtains intelligence is still intelligent. A computer program with sufficiently many conditionals is intelligent, but it never learns. An intelligent system does not have to learn. A non-learning intelligent system will continue to satisfy its goal as long as the system stays in the environments it is familiar with.

Both intelligence and learning requires measuring how well something is done.

Teaching, autodidactism, and metalearning

What is teaching? “Teacher teaches Thing to Learner” iff Teacher helps Learner learn Thing. Teaching is mostly about sequencing lessons to maximize learning speed.

What is the relationship between teaching and learning? Teachers needs learners, because otherwise there is no one to teach. If learning is the shaping of belief, then teaching is the spreading of belief. Belief is software: belief can be duplicated but not be moved. Language enables some belief transfer and capture. If we know how to learn, then we know how to teach, and also the converse. “Learn” is a transitive verb that takes one object: one learns something. “Teach” is a transitive verb that takes two objects: one teaches something to someone, possibly to thonself in case of autodidactism. But “autodidactism” (“self-teaching”) is somewhat nonsensical: you can’t tell yourself something that you don’t know. When you read a book, the book teaches you, and you learn from the book. But you may speed up your learning using some metalearning techniques. Thus, when we say “autodidactism”, we actually mean “metalearning”. Of human learning, the most important ideas seem to be goal-directed learning, the forgetting curve[fn::https://en.wikipedia.org/wiki/Forgetting_curve], and Bloom’s taxonomy of learning[fn::https://en.wikipedia.org/wiki/Bloom%27s_taxonomy].

Learning complexity

How complex is something to learn? Every computable thing is learnable, in principle. Formal language with lower descriptive complexity is more learnable. Smoother functions are more learnable. This suggests that computation theory : computation-complexity theory = learning theory : learning-complexity theory.

Smoother functions are more learnable (easier to learn). Convex boundary is more learnable than concave boundary. A polyhedron is a three-dimensional polygon. A polytope is a higher-dimensional polyhedron. The analogy is polytope : polyhedron : polygon = hypercube : cube : square. The boundary of a cluster is a polytope. A cluster with convex polytope boundary is more learnable than a cluster with concave polytope boundary.

COLT: measuring intelligence

  • Wikipedia: Computational learning theory
    • What is the goal of computational learning theory?
      • “Give a rigorous, computationally detailed and plausible account of how learning can be done.” [Angluin1992]
    • “a subfield of Artificial Intelligence devoted to studying the design and analysis of machine learning algorithms”
  • Supervised learning is extrapolating a function from finite samples. Usually, the function is high-dimensional, and the samples are few.
  • It is simple to measure learning success in perfect information games such as chess. Chess also doesn’t require any sensors and motors.

What COLT?

Optimal learning for humans https://www.kqed.org/mindshift/37289

Curate from this https://thesecondprinciple.com/optimal-learning/

Boston dynamics dog robots

Tesla car autopilots

Google and Uber self-driving cars

https://www.quora.com/Will-we-ever-have-a-rigorous-and-robust-definition-for-intelligence

rigorous definition of intelligence The new ai is general and rigorous, idsia Toward a theory of intelligence,RAND

A system responds to a stimulus. Define: a system is adapting to a stimulus if the same stimulus level elicits decreasing response level from the system. The stimulus level has to be increased to maintain the response level.

Is learning = adapting? Is intelligence = adaptiveness?

Toward a unified theory of learning?

What is learning? Shallow definitions. To learn is to avoid repeating past mistakes.

TODO Unify learning, prediction, modeling, approximation, control, hysteresis, memory. These things are similar:

  • hysteresis
  • memory
  • smoothing
  • infinite-impulse-response filter

Optimal reverse prediction unifies supervised and unsupervised learning \cite{xu2009optimal}. Then \cite{white2012generalized} generalizes \cite{xu2009optimal} to non-linear predictors.

Is hysteresis[fn::https://en.wikipedia.org/wiki/Hysteresis#Models_of_hysteresis]] learning? Is hysteresis memory? Does intelligence require learning?

Is it possible to accomplish the same goal in different environments without learning?

Use discrete sequences

Gradient descent

https://forum.azimuthproject.org/discussion/1538/machine-learning

Adversarial learning?

How do we learn amid lies, deception, disinformation, misinformation? Related to adversarial learning? https://en.wikipedia.org/wiki/Adversarial_machine_learning ?

$P$ tries to predict $G$. $G$ tries to make $P$ wrong.

Neural networks?

Neural networks is one architecture that makes machine trainable. Neural network is not necessarily the best architecture for intelligence. Evolution is a greedy optimization algorithm.

Topologically, a neural network layer is a continuous map. It transforms the input space into a more separable space. Consider the set of points that satisfy the classifier. This set is a manifold. A neural network layer stretches, rotates, manipulates that manifold. The output wants to be box-shaped. But isn’t this just the idea of Kohonen’s self-organizing maps?

Models of learning

Most mathematical statements in this chapter are to be interpreted probabilistically (truth value continuum; non-binary truth value).

There should be one theory of learning that can explain the learning done by humans, animals, plants, microbes, machines, etc.

Teaching is not a dual of learning

Both “agent X teaches agent Y Z” and “agent Y learns Z from agent X” mean the same thing: “X speeds up Y’s learning Z”.

Teaching makes learning more efficient.

A teacher multiplies a learner’s productivity. No teacher can help a learner who produces zero (a learner who is unwilling to learn).

More intrinsic model

Let \(Input\) be the agent’s input type.

Let \(Output\) be the agent’s output type.

Let \(S : Input × Output → \Real\) be the scoring function.

A learning process is a function from time to test score.

Let \(x(t)\) be the agent’s input at time \(t\).

Let \(y(t)\) be the agent’s output at time \(t\).

Let \( s(t) = S(x(t),y(t)) \).

Discrete-time learning

Let \(x\) be the agent’s input sequence where each \(x_k ∈ Input\).

Let \(y\) be the agent’s output sequence where each \(y_k ∈ Output\).

Let \(s\) be a sequence of test scores where each \(s_k = S(x_k,y_k)\).

The agent learns \(S\) iff the sequence \(s\) is increasing.

The agent is \(m\)-proficient at \(S\) after time \(t\) iff \(s_k ≥ m\) for all \(k ≥ t\).

The agent’s degree of mastery (degree of expertise) is the minimum score it can reliably achieve.

The agent’s learning rate at time \(k\) is \(r_k = s_k - sk-1\).

If there exists \(f\) such that \(S(x,y) = \norm{y - f(x)}\), then the learning problem is also an optimization problem.

Meta-learning

Meta-learning can be thought of optimization/maximization of learning rate.

Sobolev space approximation

Learning can be seen as approximation in Sobolev spaces.

(See also: approximation theory, optimization theory, and functional analysis.)

Another possibility: In 1984 Valiant proposed the PAC (probably approximately correct) learning model \cite{valiant1984theory}, but it is limited to learning propositional logic formulas. It is one piece of the theory that we need to build intelligent systems.

Convergence

Learning can be defined as convergence.

Sequence, learning, and approximation:

Here an agent is a sequence.

The agent \(a : \Nat → T\) learns the target \(t : T\) iff the sequence \(a\) converges to \(t\).

Formally, the agent \(a\) learns the target \(t\) iff \(limn→\infty a_n = t\).

Let there be a system. Devise a test. Let the system do the test several times. Let the test results be the sequence \(x\). We say that the system is getting better at that test iff, mostly, \[ i < k \implies x_i < x_k \] that is, iff the sequence of test scores is mostly increasing.

Other models of learning

(Why do we bother discussing this if we won’t use this further?) Psychology sees learning as adaptation and habituation. Formal education sees learning as getting high grades in exams. Epistemology sees learning as acquisition of knowledge. YouTube sees learning as maximizing people’s addiction to YouTube so that they linger on YouTube, with the hope that they click more ads. Each of those models is about getting better in something.

Preece 1984 \cite{preece1984mathematical}[fn::<2020-01-12> https://onlinelibrary.wiley.com/doi/pdf/10.1002/tea.3660210910]: differential equation model of learning: “Hicklin [1976] envisaged that learning resulted from a dynamic equilibrium between information acquisition and loss”.

ML stands for “machine learning”. “Machine learning addresses the question of how to build computers that improve automatically through experience.”\cite{jordan2015machine} However, we are not only interested in humans and machines, but in all intelligent beings.

Machine learning is finding a function fitting a data list, minimizing error on unseen data. Machine learning is about how program improves with experience.

Find a function fitting the data and minimizing the loss function.

Given $[(x_1,y_1),\ldots,(x_n,y_n)]$, find $f$ minimizing $∑_k \norm{f(x_k) - y_k}^2$.

A model is a constrained optimization problem: Given $C$, compute $minx ∈ C f(x)$ or $\argminx ∈ C f(x)$. If $C$ is discrete, use dynamic programming. If $C$ is continuous, use gradient descent.

A learner inhabits $[(a,b)] → (a → b)$.

A loss function inhabits $(a,b,\Real^∞) → \Real$.

The training loss of $g(x) = w ⋅ f(x)$ with respect to $D$ is $\frac{1}{|D|} ∑(x,y) ∈ D L(x,y,w)$ where $L$ is the loss function.

Learning is finding $w$ that minimizes the training loss.

Let $y ∈ \{-1,+1\}$. The score of $f$ for $(x,y)$ is $f(x)$. The margin of $f$ for $(x,y)$ is $f(x) ⋅ y$.

Binarization of $f$ is $\sgn ˆ f$.

Least-squares linear regression

Minimize training loss

Gradient descent training with initial weight $w_1$, iteration count $T$, and step size $η$: Let $K : \Real^n → \Real$ be the training loss function. Let $∇ K$ be the gradient of $K$. The weight update equation is $wt+1 = w_t - η ⋅ (∇ K)(w_t)$ where $w_1$ may be random. The training result is $w_T$.

Stochastic gradient descent (SGD) training: $wt+1 = w_t - η ⋅ (∇(L~x_t~y_t))(w_t)$. Note the usage of the loss function $L$ instead of the training loss function $K$.

SGD is online or incremental training.

Classification is regression with zero-one loss function. Every classification can be turned into regression by using hinge loss or logistic regression.

The logistic function is $f(x) = \frac{1}{1 + e-x}$.

Nearest neighbor with training data list $D$: $g(x’) = y$ where $(x,y) ∈ D$ minimizing $\norm{f(x’) - f(x)}^2$.

Seminal papers?[fn::https://www.quora.com/What-are-the-most-important-foundational-papers-in-artificial-intelligence-machine-learning]

TODO Read?

More about learning

https://en.wikipedia.org/wiki/Learning

https://en.wikipedia.org/wiki/Machine_learning

<2019-11-27> On learning, approximation, and machine learning

Approximation error \( ∑x ∈ D d(f(x),\hat{f}(x)) \) where \(d\) is the discrete metric[fn::<2019-11-27> https://en.wikipedia.org/wiki/Metric_(mathematics)#Examples][fn::<2019-11-27> https://en.wikipedia.org/wiki/Discrete_space] (equality comparison): \( d(x,y) = 0 \) iff \( x = y \) and \( d(x,y) = 1 \) iff \( x ≠ y \).

Connectionist machine learning is the art of giving machines feelings, because feelings can hardly be explained by language, which is used for thinking and not feeling.

\( I_D(x \mapsto d(f(x),\hat{f}(x))) \).

\( d \) is a distance function.

Is there machine learning on finite fields? Boolean functions? Unit interval?

\( f : D → C \).

\( f : \Real^∞ → \Real \)?

\( f : A^∞ → A \)?

\( f : A^n → A \) where \( A \) is a finite field?

Define learning. What does it mean to learn something? What does it mean to learn a function?

How do we measure generalizability?

Machine learning is about finding the shape of the approximating function?

Teaching and learning

  • How to teach history (or anything)
    • Don’t memorize things that you can look up on the Internet.
    • Focus on stories, insights, reasons, motivations.
    • Empathize with the subjects. Why do they go to war?
  • Learning languages, both human languages and programming languages
    • One learns a language by example sentences. One learns a programming language by example programs/snippets.
      • One does not learn a language by memorizing the syntax.
      • One does not learn a language by memorizing the language reference document.

Bibliography