Towards an Objective Theory of Rationality

6. A Realist Theory of Knowledge

6.2 Perceptual Effects from Subjective Influences

Book cover: The Structure of Science by Ernest Nagel

Turning now to the second complication for an empiricist view of observation, scientific research reveals that what we perceive visually is not simply a two-dimensional retinal image. What we see is in fact the result of complex neurophysiological processing of the visual information that enters our eyes mediated by our current theoretical construct of the world. Cognitive science reveals that there are ubiquitous active top-down feedback networks that impact the bottom-up processing of our sensory inputs.[20] The nature of this processing means that our phenomenological experience is not only a function of the image cast on our retina, but also of our genetic constitution,[21] our prior beliefs and our expectations. Our expectations serve to influence our perception of colour, shape, speed of motion, brightness and distance, among other things.[22]

For example, in one study, Jones and Bruner [1954] showed experimental subjects a sequence of cards depicting a 'stick-man' running and a nonsense object moving. The observers reported perceiving the 'stick-man' moving faster and travelling a greater distance than the nonsense object, even though in actual fact they had both traversed the same distance in the same amount of time. The running 'stick-man' was seen to move faster and further because it was expected to move faster than an object that had no significance for the experimental observers.

Such expectations and, consequently, perceptions can also be influenced by suggestion. In another study [Hastorf 1950], experimental subjects perceived the same white rectangle to be at a further distance from them when they were informed that it was an envelope compared with when they were told that it was a calling card. Similarly, a white circle was perceived as being located further away when it was suggested that it was a billiard ball compared with when it was suggested that it was a ping pong ball.[23]

These experiments demonstrate that our expectations are dependent on our prior experiences and beliefs. However, our prior experiences and beliefs can also shape our perceptual experiences in the absence of accompanying expectations. For example, Steinfeld [1968] demonstrated how a group of experimental subjects having been first read a story about a ship more quickly and easily recognized a fragmented drawing of a ship compared with an otherwise similar group that had not.[24]

Research studies have also suggested that in some circumstances our perceptions are partly shaped by our needs, values and personalities.[25] Not only are our perceptual experiences influenced by our internal subjective states, but our recollections of our perceptual experiences can also be distorted by our previous beliefs and later experiences. For example, many studies have confirmed that our memories can be easily modified by later suggestions.[26] It is not only under abnormal conditions that our perceptual experiences and memories are shaped by our inner subjective states. Some of these experimental studies indicate that this happens under normal conditions as well.

The conclusion we must draw from these research studies is that we need to treat observational evidence presented in the form of personal anecdotes with due caution. We have already noted at the beginning of this section that observation statements about the external world are interpreted in the light of particular theories expressible in a public language about physical objects. We can also now add the insight that these same observation statements are based on phenomenological sense-experiences that can be easily distorted by the observer's psychological state at the time and on memories that can be shaped by later psychological states.


  1. [20] For an overview of research on top-down processing of stimuli, see Kveraga et al [2007] and Bubic et al [2010].
  2. [21] See §5 above for the role played by the hard-wiring in our visual cortex in our perception of depth and contour.
  3. [22] There is now a wealth of literature on this subject. For a reasonably comprehensive survey of the literature, see Vernon [1971]. For a selection of experimental reports, see the Psychology of Perception section under References at the end of this essay.
  4. [23] For other studies on the influence of suggestion on perception, see, for example, Landauer and Rodger [1964] and McGee [1963].
  5. [24] See also, for example, Borresen and Lichte [1962] and Proshansky and Murphy [1942].
  6. [25] See Vernon [1971: chs 10, 11], Comalli Jr [1960], Levine, Chein and Murphy [1942], Smith, Parker and Robinson Jr [1951] and Tajfel [1957].
  7. [26] See, for example, Loftus [1979].

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