First Distinction

As early as 1890, two different types of memory were described in the book ‘Principles of Psychology’ by William James. He distinguished:

• The primary memory, or the stream of consciousness. Today, this is referred to as the short-term memory or working memory.
• The secondary memory, or the memory for the past. Today, this is referred to as the long-term memory.

Dividing the Long-Term Memory

As time passed, it became apparent that the long-term memory could be further divided, into:

• The declarative memory, which is responsible for the conscious memories of facts and events.
• The non-declarative memory, which is responsible for unconscious skills that are expressed in the behavior.

Dividing the Non-Declarative Memory

The non-declarative memory can be further divided into two types:

• The procedural memory, which stores knowledge of how to do things. It is responsible for remembering how to ride a bike, tie our shoelaces, swim, and so on. These are relatively complex skills to learn, but once we have mastered them, we can do them almost automatically and we do not lose the ability to do them.
• The implicit memory, which contains more than purely knowledge of certain physical acts. Here, all memories that do not require conscious processing are stored, such as perceptual learning (learning by seeing), priming (unconscious directing of attention) and implicit conditioning.

Dividing the Declarative Memory

Lastly, the declarative memory can also be further divided into two types:

• The episodic memory, which is the memory where the events one has experienced are stored. Memories stored here contain information about the time and place of a certain event (such as meeting someone, buying something, and so on).
• The semantic memory, which is where facts and knowledge about the world are accumulated. Memories contained here, for example, are knowing that wood burns, that one and one makes two, that Paris is the capital of France, and so on. In this type of memories, the time or place of acquiring them is of no real importance, and thus this is usually not stored. Do you, for example, remember the exact time and place where you first learned that wood can burn?

References

• Baddely, A. (1997). Human Memory: Theory and Practice. Psychology Press, UK.
• Izquierdo, I.; Barros, D.M.; Mello e Souza, T.; de Souza, M.M.; Izquierdo, L.A. & Medina, J.H. (1998). Mechanisms for memory types differ. Nature, 393, pp. 635 – 636.
• Tulving, E. & Schacter, D.L. (1990). Priming and Human Memory Systems. Science. 247(4940), pp. 301 – 306.