The Nine Senses!

Twenty-four centuries ago, Aristotle insisted there were only five senses. Boy, was he wrong! Image by Chubykin Arkady/Shutterstock

18 October 2023 – We owe the original list of five senses to Aristotle, who said “… There is no sixth sense in addition to the five enumerated — sight, hearing, smell, taste, touch …” (Aristotle, 1931). For nearly two millennia, folks labored under this delusion, but by the 16th century natural philosophers (the elite scientists of the day) had started to doubt the completeness of Aristotle’s list. In 1557 physician Julius Caesar Scaliger described a sixth sense kinesthesia, or proprioception, as the internal perception of body position. More recently, scientists have described other senses, extending the list until it has grown to eight:

  1. Vision (sight);
  2. Audition (hearing);
  3. Olfaction (smell);
  4. Gustation (taste);
  5. Haptics (touch);
  6. Kinesthesia (body position, muscle effort);
  7. Vestibular (sense of gravity and acceleration);
  8. Interoception (physiological state of the body).

This is currently the list as quoted by most scientists and practitioners interested in the science of sentience and cognition. I got serious about sense enumeration while reading Andy Clark’s new book The experience machine: How our minds predict and shape reality (Clark, 2023). The book is important reading for anyone interested in the science of cognition, but a complete description in this blog will have to wait until I get around to writing a full critique of it. This present column is about the sense of time, which is a relatively recent addition to the growing list of human senses, bringing it up to nine.

The roughly 1,900-year lag between Aristotle’s original work and the addition of time sense to the list of human perceptions is a tribute to both that ancient Greek’s domination of early scientific thought and his habitual sloppy interpretation of observations. Notwithstanding, we talk about time sense constantly. Time is embedded in our language and is fundamental to our understanding of the Universe. Indeed, Newton began his The mathematical principles of natural philosophy, which is generally acknowledged to be the first complete description of Classical (Newtonian) Physics, by postulating an absolute time that is the same for everyone everywhere (Newton, 2002).

It turns out that my former ignorance of any scholarly literature exploring the temporal sense as a feature of the human sensory system is another example of the truth of the aphorism (often apocryphally attributed to Carl Sagan): “Absence of evidence is not evidence of absence.” A more careful literature search turned up scholarly articles on the subject reaching back to the late 1980s (Czeisler et al., 1999). That, of course, ignores the even older recognition of longer physiological cycles, such as the circadian rhythm common in animals and plants, which has been carefully studied since the middle of the 20th century (Aschoff, 1965).

One possible reason that the temporal sense has been largely ignored by many researchers is that there seems to be no single, dedicated internal temporal organ serving up time sensation for the entire body. Vision has the eyes. Audition has the ears. Where is the clock for a temporal sense? Instead, many organs, such as the heart, seem to have independent pacemakers for their own activities (Schulz & Steimer, 2009).

Another potential reason scholars have ignored the time sense is the recognition that sentient beings receive abundant clues about the passage of time from their surroundings. This could, the thinking goes, obviate the need for an internal temporal sense organ. Who needs an internal clock to know whether it is day or night when Nature makes it obvious through other senses? The difference between having a time sense and just relying on so-called zeitgebers—external cues to the passage of time (Schulz & Steimer, 2009) is whether one can sense time passage when cut off from other sensory input from outside. There is now abundant research showing that humans (at least) sense time accurately even under conditions of intense sensory deprivation (Aschoff, 1965).

Temporal Cycles

The most important aspect of time for a mobile sentient creature (i.e., an animal) is the notion of cycles. The three cycles that are most significant to terrestrial creatures are the annual, lunar and solar (diurnal), which repeat on a yearly, monthly and daily period respectively. Animals (and plants, but we’re here just considering animals, and specifically Homo sapiens sapiens) need to key their activities to these three cycles because they so dramatically affect environmental conditions, which, in turn, affect opportunities for activities.

Chronobiologists (scientists who study the temporal aspects of biological systems) recognize three types of temporal cycles: ultradian, circadian, and infradian based on their frequency, or the number of beats per unit time. For biologic systems, the most salient seem to be circadian rhythms, which have a frequency of approximately one cycle per day. Infradian rhythms are slower (lower frequency), such as the menstrual cycle. Ultradian rhythms, such as the heartbeat, are faster. The whole range of frequencies important to biologic systems extends from microseconds to multiple decades (Czeisler, et al., 1999; Honma, et al., 2023). Two especially important brain structures for time sensing are the suprachiasmatic nucleus (SCN), which provides a pacemaker for circadian clocks in mammals (Schulz & Steimer, 2009), and the subthalamic nucleus (STN), which provides a pacemaker for judging time intervals shorter than a few minutes (Honma, et al., 2023). These are both structures centrally located in the human brain.

Ready, SET, Go!

SET is an acronym for scalar expectancy theory, which is a framework that describes how the brain might use the time sense to compare a presently experienced time interval to a previously experienced time interval, such as, say, the time between breakfast and lunch (Honma, et al., 2023). SET postulates a clock made up of a pacemaker, a switch, and an accumulator. The pacemaker provides a free-running oscillatory signal at a frequency appropriate for physiological functions using the time sense (a few tens of Hertz). The switch gates clock pulses from the pacemaker into the accumulator, which keeps a running count of pulses in the brain’s short-term working memory. The brain then compares the accumulator’s current contents to the expected number recalled from the brain’s reference memory of previous occurences of the event. An individual experiences a current time interval of interest as a fraction of the expected time interval recalled from reference memory. As the ratio approaches unity, the individual experiences a growing sense of anticipation that it’s time for the waiting interval to end. If the ratio exceeds unity without the waited-for event happening, the individual begins to wonder why it is taking so long.

Similar or equivalent mechanisms likely provide mechanisms for the brain to develop a feeling that it knows what time it is in the real world and can feel the passage of time. This is the temporal equivalent of the conscious experience of being able to see how far it is to that oncoming car, or that the ambient temperature is uncomfortably hot this afternoon.


Aristotle. (1931). De Anima (J. A. Smith, trans.) Classics in the History of Psychology. (Original work published ca. 350 BCE)

Aschoff, J. (1965). Circadian rhythms in man. Science. 148(3676) 1427–32.

Clark, A. (2023). The experience machine: How our minds predict and shape reality. Pantheon Books.

Czeisler, Charles & Duffy, Jeanne & Shanahan, Theresa & Brown, Emery & Mitchell, Jude & Rimmer, David & Ronda, Joseph & Silva, Edward & Allan, James & Emens, Jonathan & Dijk, Derk-Jan & Kronauer, Richard. (1999). Stability, Precision, and Near-24-Hour Period of the Human Circadian Pacemaker. Science. 284. 2177-2181.

Schulz, P., & Steimer, T. (2009). Neurobiology of Circadian Systems. CNS Drugs, Suppl.Supplement 2, 23, 3-13.

Honma, M., Sasaki, F., Kamo, H., Nuermaimaiti, M., Kujirai, H., Atsumi, T., Umemura, A., Iwamuro, H., Shimo, Y., Oyama, G., Hattori, N., & Terao, Y. (2023). Role of the subthalamic nucleus in perceiving and estimating the passage of time. Frontiers in Aging Neuroscience. 15 DOI: 10.3389/fnagi.2023.1090052

Newton, I. (2002). The mathematical principles of natural philosophy (D. R.Wilkins, Ed., A. Motte, Trans.). Daniel Adee Publisher. (Original work published 1729).

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