Posted by: Xhyra Graf | 20 January 2007

Sense Data Sense data are the (alleged) mind-dependent objects that we are directly aware of in perception, and that have exactly the properties they appear to have.

Critics of sense data have objected to the theory’s commitment to mind-body dualism, its difficulty in locating sense data in physical space, and its apparent commitment in some cases to sense data that have indeterminate properties. The term ‘sense data’ (singular: ‘sense datum’) was introduced by early 20th century philosophers such as H. H. Price, G. E. Moore, and Bertrand Russell, with the stipulation that it should refer to that which we are directly aware of in perception. Originally, the term’s meaning was supposed to be neutral between competing theories of perception, so that it was not to be assumed either that sense data must (analytically) be mind-dependent or that they must be mind-independent. However, those who used the term came so consistently to the conclusion that ‘sense data’ are mind-dependent that the term has come, in contemporary philosophy, to include an assumption of mind-dependence as part of its meaning. ‘Sense data’ now means an (alleged) kind of thing that

  1. we are directly aware of in perception,
  2. is dependent on the mind, and
  3. has the properties that perceptually appear to us.

Each of those conditions calls for clarification.

The sense data theory contrasts with two competing schools of thought in the philosophy of perception. First, direct realism holds that in perception, we are directly aware (only) of things in the physical world–for example, a table, or a portion of a table’s surface. Direct realists thus deny that there is anything satisfying both conditions (i) and (ii) above, and therefore deny that there are sense data. Second, the adverbial theory (in one version) holds that in perception, we are directly aware of a certain kind of mental state or occurrence, but that this mental state does not actually possess the properties that appear to us. Adverbialists have been known to characterize this mental state in such terms as “being appeared to redly.” When a person is in the mental state of being appeared to redly, say the adverbialists, it does not follow that anything is actually red. Thus, adverbialists deny that there is anything satisfying all of conditions (i), (ii), and (iii), and therefore deny that there are sense data.

2. Arguments for Sense Data

Perspectival variation is the kind of variation in one’s sensory experiences that normally attends changes in one’s spatial or other physical relationship to the physical objects one is observing. Perspectival variation, in this sense, is ubiquitous. For instance, suppose you are viewing a table. If you move closer to or farther from the table, your sensory experience changes (Hume [1758, XII.1] described this change, perhaps incorrectly, by saying that the table “seems to diminish” as you move away from it). If you move laterally, relative to the table, your sensory experience will change in another way (Russell [1997, pp. 10-11] describes this phenomenon by saying that the tabletop appears different shapes when viewed from different angles).

Some have argued that the phenomenon of perspectival variation shows that what we are immediately aware of in perception cannot be the real, external objects, but must instead be only images in the mind (sense data). Here is one way of making out that argument:

  1. In the phenomenon of perspectival variation, the thing we are directly aware of changes (for instance, its size or shape changes).
  2. The real, external object is not changing at this time.
  3. Therefore, the thing we are directly aware of is not the real, external object.

An illusion is a case in which one perceives an object, but the object is not the way it appears in some respects. For instance, when one views a straight stick half-submerged in water, the stick may appear bent. Since it is not in fact bent, this is an illusion.

Some philosophers have argued that the possibility of sensory illusions shows that what we are directly aware of in perception is never the real, physical object. Using the bent-stick illusion as an example, one might argue:

  1. When viewing a straight stick half-submerged in water, one sees (or is directly aware of) something bent.
  2. No relevant physical thing is bent in this situation.
  3. Therefore, in this situation, one sees something non-physical.
  4. What one sees in this situation is the same kind of thing that one sees in normal (non-illusory) perception.
  5. Therefore, in normal perception, one sees non-physical things.

A background assumption is that there is only one stick-like thing that one sees in the example, and that thing is either an actual, physical stick, or a mental image of a stick (a sense datum). The argument concludes that it is not the physical stick, so it must be a sense datum.

A hallucination is a case in which one has an experience qualitatively like perception, but there is no external object that one is perceiving. For instance, an overdose of LSD might cause me to have an experience of seeming to see a pink rat on this table, where there is in reality nothing pink-rat-like.

The intended argument may be something like this:

  1. In a case of double vision, one sees two of something.
  2. There are not two (relevant) physical objects in this situation.
  3. Therefore, in a case of double vision, one sees something non-physical.

There is always a time delay between any event in the physical world and our perception of it. This is most stark in the case of distant stars, which may burn out and yet still be ‘seen’ thousands of years later, as the light continues to travel the distance between the star and us.

Many philosophers have held that the so-called ‘secondary qualities’ — including such qualities as colors, tastes, smells, and sounds — do not exist in the external world, and that we must instead recognize them as properties of sense data. Consider the case of colors. A sense data theorist might argue:

  1. Everything we (directly) see has color.
  2. No physical thing is colored.
  3. Therefore, everything we (directly) see is non-physical

The first premise seems obvious on its face. The second premise may seem unbelievable, but there are several arguments for it. One of these arguments appeals to differences of color perception among people.

3. Objections to Sense Data





Huemer, Michael, “Sense-Data”, The Stanford Encyclopedia of Philosophy (Spring 2005 Edition), Edward N. Zalta (ed.), URL =


  1. […] How these all are affected by [Is that right? Maybe-how they fit into the conceptual scheme that includes] the specious present and a certain view of sense data. […]


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