we have access to ideas, or mental representations of the external world, but not direct access to the world itself." thus in order to keep these ideas organized, attention is necessary. otherwise we will confuse these ideas. malebranche writes in "the search after truth", "because it often happens that the understanding has only confused and imperfect perceptions of things, it is truly a cause of our errors.... it is therefore necessary to look for means to keep our perceptions from being confused and imperfect. and, because, as everyone knows, there is nothing that makes them clearer and more distinct than attentiveness, we must try to find the means to become more attentive than we are". according to malebranche, attention is crucial to understanding and keeping thoughts organized. philosopher gottfried wilhelm leibniz introduced the concept of apperception to this philosophical approach to attention. apperception refers to "the process by which new experience is assimilated to and transformed by the residuum of past experience of an individual to form a new whole." apperception is required for a perceived event to become a conscious event. leibniz emphasized a reflexive involuntary view of attention known as exogenous orienting. however, there is also endogenous orienting which is voluntary and directed attention. philosopher johann friedrich herbart agreed with leibniz's view of apperception; however, he expounded on it in by saying that new experiences had to be tied to ones already existing in the mind. herbart was also the first person to stress the importance of applying mathematical modeling to the study of psychology. throughout the philosophical era, various thinkers made significant contributions to the field of attention studies, beginning with research on the extent of attention and how attention is directed. in the beginning of the 19th century, it was thought that people were not able to attend to more than one stimulus at a time. however, with research contributions by sir william hamilton, 9th baronet this view was changed. hamilton proposed a view of attention that likened its capacity to holding marbles. you can only hold a certain number of marbles at a time before it starts to spill over. his view states that we can attend to more than one stimulus at once. william stanley jevons later expanded this view and stated that we can attend to up to four items at a time. 1860–1909 this period of attention research took the focus from conceptual findings to experimental testing. it also involved psychophysical methods that allowed measurement of the relation between physical stimulus properties and the psychological perceptions of them. this period covers the development of attentional research from the founding of psychology to 1909. wilhelm wundt introduced the study of attention to the field of psychology. wundt measured mental processing speed by likening it to differences in stargazing measurements. astronomers in this time would measure the time it took for stars to travel. among these measurements when astronomers recorded the times, there were personal differences in calculation. these different readings resulted in different reports from each astronomer. to correct for this, a personal equation was developed. wundt applied this to mental processing speed. wundt realized that the time it takes to see the stimulus of the star and write down the time was being called an "observation error" but actually was the time it takes to switch voluntarily one's attention from one stimulus to another. wundt called his school of psychology voluntarism. it was his belief that psychological processes can only be understood in terms of goals and consequences. franciscus donders used mental chronometry to study attention and it was considered a major field of intellectual inquiry by authors such as sigmund freud. donders and his students conducted the first detailed investigations of the speed of mental processes. donders measured the time required to identify a stimulus and to select a motor response. this was the time difference between stimulus discrimination and response initiation. donders also formalized the subtractive method which states that the time for a particular process can be estimated by adding that process to a task and taking the difference in reaction time between the two tasks. he also differentiated between three types of reactions: simple reaction, choice reaction, and go/no-go reaction. hermann von helmholtz also contributed to the field of attention relating to the extent of attention. von helmholtz stated that it is possible to focus on one stimulus and still perceive or ignore others. an example of this is being able to focus on the letter u in the word house and still perceiving the letters h, o, s, and e. one major debate in this period was whether it was possible to attend to two things at once (split attention). walter benjamin described this experience as "reception in a state of distraction." this disagreement could only be resolved through experimentation. in 1890, william james, in his textbook the principles of psychology, remarked: everyone knows what attention is. it is the taking possession by the mind, in clear and vivid form, of one out of what seem several simultaneously possible objects or trains of thought. focalization, concentration, of consciousness are of its essence. it implies withdrawal from some things in order to deal effectively with others, and is a condition which has a real opposite in the confused, dazed, scatterbrained state which in french is called distraction, and zerstreutheit in german. james differentiated between sensorial attention and intellectual attention. sensorial attention is when attention is directed to objects of sense, stimuli that are physically present. intellectual attention is attention directed to ideal or represented objects; stimuli that are not physically present. james also distinguished between immediate or derived attention: attention to the present versus to something not physically present. according to james, attention has five major effects. attention works to make us perceive, conceive, distinguish, remember, and shorten reactions time. 1910–1949 during this period, research in attention waned and interest in behaviorism flourished, leading some to believe, like ulric neisser, that in this period, "there was no research on attention". however, jersild published very important work on "mental set and shift" in 1927. he stated, "the fact of mental set is primary in all conscious activity. the same stimulus may evoke any one of a large number of responses depending upon the contextual setting in which it is placed". this research found that the time to complete a list was longer for mixed lists than for pure lists. for example, if a list was names of animals versus a list of the same size with names of animals, books, makes and models of cars, and types of fruits, it takes longer to process the second list. this is task switching. in 1931, telford discovered the psychological refractory period. the stimulation of neurons is followed by a refractory phase during which neurons are less sensitive to stimulation. in 1935 john ridley stroop developed the stroop task which elicited the stroop effect. stroop's task showed that irrelevant stimulus information can have a major impact on performance. in this task, subjects were to look at a list of colors. this list of colors had each color typed in a color different from the actual text. for example, the word blue would be typed in orange, pink in black, and so on. example: blue purple red green purple green subjects were then instructed to say the name of the ink color and ignore the text. it took 110 seconds to complete a list of this type compared to 63 seconds to name the colors when presented in the form of solid squares. the naming time nearly doubled in the presence of conflicting color words, an effect known as the stroop effect. 1950–1974 in the 1950s, research psychologists renewed their interest in attention when the dominant epistemology shifted from positivism (i.e., behaviorism) to realism during what has come to be known as the "cognitive revolution". the cognitive revolution admitted unobservable cognitive processes like attention as legitimate objects of scientific study. modern research on attention began with the analysis of the "cocktail party problem" by colin cherry in 1953. at a cocktail party how do people select the conversation that they are listening to and ignore the rest? this problem is at times called "focused attention", as opposed to "divided attention". cherry performed a number of experiments which became known as dichotic listening and were extended by donald broadbent and others.: 112 in a typical experiment, subjects would use a set of headphones to listen to two streams of words in different ears and selectively attend to one stream. after the task, the experimenter would question the subjects about the content of the unattended stream. broadbent's filter model of attention states that information is held in a pre-attentive temporary store, and only sensory events that have some physical feature in common are selected to pass into the limited capacity processing system. this implies that the meaning of unattended messages is not identified. also, a significant amount of time is required to shift the filter from one channel to another. experiments by gray and wedderburn and later anne treisman pointed out various problems in broadbent's early model and eventually led to the deutsch–norman model in 1968. in this model, no signal is filtered out, but all are processed to the point of activating their stored representations in memory. the point at which attention becomes "selective" is when one of the memory representations is selected for further processing. at any time, only one can be selected, resulting in the attentional bottleneck.: 115–116 this debate became known as the early-selection vs. late-selection models. in the early selection models (first proposed by donald broadbent), attention shuts down (in broadbent's model) or attenuates (in triesman's refinement) processing in the unattended ear before the mind can analyze its semantic content. in the late selection models (first proposed by j. anthony deutsch and diana deutsch), the content in both ears is analyzed semantically, but the words in the unattended ear cannot access consciousness. lavie's perceptual load theory, however, "provided elegant solution to" what had once been a "heated debate".