21. Probabilistic Inference I

Probabilities in Artificial Intelligence

With a joint probability table, recording the tally of crossed events occurrence will allow us to measure the probabilities of each event happening, conditional or unconditional probabilities, independence of events, etc.

The problem with such table is that as the number of variables increase, the number of rows in the table grows exponentially.

Reminders of probabilities formulas

Basic axioms of probability

  • 0 ≤ P(a) ≤  1
  • P(True) = 1 ; P(False) = 0
  • P(a+b) = P(a) + P(b) – P(a,b)

Basic definitions of probability

  • P(a/b) = P(a,b) / P(b)
  • P(a,b) = P(a/b) P(b)
  • P(a/b,c) = P(a/b,c) P(b,c) = P(a/b,c)P(b/c)P(c)

Chain rule of probability

By generalizing the previous formula, we obtain the following chain rule:

Chain rule of probability


Independent events

P(a/b) = P(a) if a and b are independent

Conditional independence

If a and b are independent

  • P(a/b+z) = P(a/z)
  • P(a+b/z) = P(a/z)P(b/z)

Belief nets

Causal relations between events can be represented in nets. These models highlight that any event is only dependent from its parents and descendants. Recording the probabilities at each node, the number of table and rows is significantly smaller than a general table of all events tallies.

19. Architectures: GPS, SOAR, Subsumption, Society of Mind

General Problem Solver

By analyzing the difference between a current state and desired state, a set of intermediary steps can be created to solve the problem = problem solving hypothesis.

SOAR (State Operator And Result)

SOAR Components:

  • Long-term memory
  • Short-term memory
  • Vision system
  • Action system

Key parts of the SOAR architecture:

  1. Long-term memory and short-term memory
  2. Assertions and rules (production)
  3. Preferences systems between the rules
  4. Problem spaces (make a space and search through that space)
  5. Universal sub-goaling: new problems that emerge during the resolution become entire new goal with assertions rules, etc.

SOAR relies on the symbol system hypothesis. It primarily deals with deliberative thinking.

Emotion machine

Created by Marvin Minsky to tackle more complex problems, this architecture involves thinking about several layers:

  • Reflective thinking
    • Self-conscious
    • Self-reflective
  • Deliberative thinking
  • Learned reaction
  • Instinctive reaction

It is based upon the common sense hypothesis.


System created by Rodney Brooks. By generalizing layers of abstraction in the building of robots (such as for robot vision and movement), modifications to certain layers don’t interfere with other layers computation, allowing for better incremental improvement of the system as a whole.

It primarily deals with instinctive reaction and learned reaction.

This is the creature hypothesis, if a machine can act as an insect, then it will be easy to develop further later. This architecture relies upon the following principles:

  1. No representation
  2. Use the world instead of model: reacting to the world constantly
  3. Finite state machines


Based upon language, this system involves perception and description of events, which then allow to understand stories and further, culture both at the macro (country, religion…) and micro (family…) levels. This system relies upon the strong story hypothesis.

18. Representations: Classes, Trajectories, Transitions


In a semantic net, a diagram of relations between objects, essential notions can be defined as follows:

  • Combinators: linking objects together
  • Reification: actions implying results
  • Localization: a frame where objects and actions happen
  • Story sequence: a series of actions happening linearly in time


In natural language, knowledge is generally organized from general categories to basic objects and finally specific objects.


Another element of language is recording change in the evolution of objects during the unfolding of stories.


Language also tracks movement in the description of actions.

An agent makes an object move from a source to a destination using a instrument. A co-agent may be involved, the action might be aimed towards a beneficiary, and the motion may be helped by a conveyance, etc. In English, preposition tend to be used to define the role of each part in the action, enabled recording of interactions.

Language corpuses, such as the Wall Street Journal Corpus, are generally composed of 25% of transition or trajectory.

Story sequences

Agents’ action determine transitions in the semantic net, which result in trajectories.

Story libraries

Each type of story implies a number of characteristics that correspond to the situation. Example: events can be disasters or parties, they have a time and place, involved people, casualties, money, places…

17. Learning: Boosting

“Wisdom of a weighted crowd of experts”


Classifiers are tests that produce binary choices about samples. They are considered strong classifiers if their error rate is close to 0,  weak classifiers if their error rate is close to 0.5.

By using multiple classifiers with different weights, data samples can be sorted or grouped according to different characteristics.

Decision tree stumps

Aside from classifiers, a decision tree can be used to sort positive and negative samples in a 2-dimension space. By adding weights to different tests, some samples can be emphasized over the others. The total sum of weights must always be constrained to 1 to ensure a proper distribution of samples.

Dividing the space

By minimizing the error rate of the tests from the weights, the algorithm can cut the space to sort positive and negative examples.

No over fitting

Boosting algorithms seems not to be over fitting, as the decision tree stumps tends to be very tightly close to outlying samples, only excluding them from the space.

16. Learning: Support Vector Machines

Decision boundaries

Separating positive and negative example with a straight line that is as far as possible from both positive and negative examples, a median that maximizes the space between positive and negative examples.

Constraints are applied to build a support vector (u) and define a constant b that allow to sort positive examples from negative ones. The width of a “street” between the positive and negative values is maximized.

Going through the algebra, the resulting equation show that the optimization depends only on the dot product of pair of samples.

The decision rule that defines if a sample is positive or negative only depends on the dot product of the sample vector and the unknown vector.

No local maximum

Such support vector algorithm can be proven to be evolving in a convex space, meaning that it will never be blocked at a local maximum.

Non linearity

The algorithm cannot find a median between data which cannot be linearly separable. A transformation can however be applied to the space to reorganize the samples so that they can be linearly separable. Certain transformations can however create an over fitting model that becomes useless by only sorting the example data.