In the mediaeval system of psychology developed by, inter alia, St Thomas Aquinas and Avicenna, the vis aestimativa is a part of the cognitive apparatus possessed by humans, mammals and the higher animals.

Put simply it is the ability to assess all the sensory products and assemble them into a meaningful conclusion. The raw sensations of the five senses (touch, sight, smell, sound, and taste) are unified and processed by the sensus communis (the 'internal sense'), and passed to the vis aestimativa. The vis aestimativa in turn is able to build on this sensory data, adding assessment based on instinct or memory, allowing for apprehensions that go beyond sensory perception. It should be noted that this is not merely an aggregation of said sensory and memorative data.

Examples of the vis aestimativa could range from the instinctual 'knowledge' as to whether a person, place or thing is good or evil, to more complex assessments of need and desire. Think also of illiterate shepherds, who--despite being unable maybe to count--can sense that a sheep is missing.

It should be contrasted with the vis cogitativa, the power for strict cognition, possessed by humans alone. Animals cannot possess this, because whilst they can recognise concrete relationships--including spatial relationships--they cannot recognise abstract ones. It is the ability for sheer abstraction that distinguishes humans and the rest of the animal kingdom.

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