One of the central principles of systemics applied in the symbolic approach to cognitive science is that (1) there are different Levels of Analysis (LOA) from which the brain and mind can be studied, and (2) mental phenomena are best studied from multiple levels of abstraction. For example, these levels are broken into three (not well separated) groups, based on Marr's description of them:

Implementational (Physical) level: describes the physical substrate that the system consists of (e.g. the brain; neurons).

Algorithmic (Functional) level: describes how information is processed to produce the behavioral output.

Computational (Behavioral) level: describes the directly observable output (or behavior) of a system.

A simple analogy often used to describe LOA is to compare the brain to a computer. The physical level would consist of the computer's CPU, the behavioral level represents the computer's output to a monitor or printer, and the functional level would be the computer's operating system, which allows the CPU and peripheral components to communicate.

A central tenet of cognitive science is that a complete understanding of the mind/brain cannot be attained by studying only a single level. For example, consider the problem of remembering a phone number and recalling it later. How does this process occur? One approach would be to study behavior through direct observation. A person could be presented with a phone number, asked to recall it after some delay. Then the accuracy of the response could be measured. Another approach would be to study the firings of individual neurons while a person is trying to remember the phone number. Neither of these experiments on their own would fully explain how the process of remembering a phone number works. Even if the technology to map out every neuron in the brain in real-time were available, and it were known when each neuron was firing, it would still be impossible to know how a particular firing of neurons translates into the observed behavior. Thus an understanding of how these two levels relate to each other is needed. This can be provided by a functional level account of the process. Studying a particular phenomenon from multiple levels creates a better understanding of the processes that occur in the brain to give rise to a particular behavior. For criticisms of this framework see Functionalism (psychology).