Some of the earliest systematic studies of thought and the accompanying desire to measure it came from astronomy, not psychology or philosophy. From this beginning, Dutch physiologist Frans C. Donders set out specifically to measure a sequence of mental process—thought—in the middle of the nineteenth century. His technique was simple yet elegant in its ability to measure how much time mental processes consume; the procedure developed by Donders is typically referred to as the subtraction technique.
The subtraction technique begins with the timing and measurement of a very basic task. For example, a person might be asked to press a button after hearing a tone. Donders realized that it was fairly easy to time accurately how long subjects took to perform this task. He believed that two cognitive (thought) processes would be operating: perception of the tone and the motor response of pressing the button. Once the time of this simple task was known, Donders would make the task more difficult. If a discrimination task were added, he believed, the time taken to complete the task would increase compared to the basic perception-motor response sequence. In this discrimination task, for example, Donders might tell a person to press the button only after hearing a high-pitched sound. That person is now faced with an added demand—to make a decision about pitch. Donders believed that with this discrimination stage, the processing of the information would require more mental effort and more time; he was right. More important, Donders could now measure the amount of extra thought required for the decision by subtracting the simple-task time from the discrimination-task time. In a general sense, Donders had a method for measuring thought.
Donders also had the ability to measure and manipulate specific components of the thought process. He even added another component to the sequence of tasks, what he called choice time. For example, the task could be changed so that for a high tone the subject should press the right button and for a low tone press the left button. By subtracting the discrimination time from this new choice time, he could estimate how long the added choice contributed to the overall thought process. By means of these inge nious methods, Donders inspired generations of cognitive psychologists to study thought in terms of the time it takes to think.
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