Everyone seems to produce such slips of the tongue. Sigmund Freud was aware of this fact and in 1901 based his monograph, Psychopathology of everyday life, on such errors claiming that slips of the tongue resulted from repressed thoughts which are revealed by the particular errors which a speaker makes. While it is possible that Freud is correct in some cases, such errors reveal as much if not much more about the structure of language as they do about repressed thoughts.
Linguists have collected and analyzed slips of the tongue at least as far back as the 8th century when the Arab linguist Al-Ki-sa-i wrote his book, Errors of the populace. The Arab scholar's interest in such errors was based on the belief that they might provide clues as to how language changes. Since that time, many linguists have been collecting and analyzing these spontaneously produced speech errors. While slips of the tongue have not contributed much to our understanding of how and why languages change, they do tell us a great deal about what we seem to know about our language and how we use this knowledge to speak and to understand what others say to us. We can look at some speech errors and see what they reveal about our linguistic knowledge.
Slips of the Tongue and the Sounds of Language
It should first be noted that the sounds we produce when we speak and those we hear when we understand speech are continuous. That is, when we say the word d o g we do not say D and then stop and say O and then G. We produce one continuous sound 'dog'. This is also true of entire sentences. Try reading the last sentence and note that you will not even pause between each word. Yet, for thousands of years, the scholars and philosophers interested in the nature of language have believed that language and speech are composed of discrete units of sound and meaning. Although the sound represented by the letters d-o-g may be continuous on a physical level, the word can be considered to be composed of separate sounds. This is as true of languages without a written alphabet (and there are thousands of such languages spoken in the world) as those like English with a written form. While these units are not normally observed in error free speech, speech errors which move or substitute, delete or add sounds or words or phrases show the existence of such units, as illustrated in the the following examples:
(The intended utterance occurs on the left side of the arrow and the actual utterance with the error on the right. The units that are involved in the errors appear in bold type. The kind of discrete unit and the mechanism which produced the error is given in parentheses.)
(1) stick in the mud > smuck in the tid (consonant segments exchange)
(2) ad hoc > odd hack (vowel segments exchange)
(3) unanimity > unamity (syllable deleted)
(4) easily enough > easy enoughly (suffix moved)
(5) tend to turn out > turn to tend out (words exchange)
(6) my sister went to the Grand Canyon > the grand canyon went to my sister (whole phrase exchange)
Slips of the Tongue and the Mental Dictionary
Speech errors also tell us a great deal about the structure and organization of the mental dictionary--the storage house of all the words a speaker of a language knows. Consider the following errors in which an unintended word substitutes for the intended one:
(7) he's going up town > he's going down town
(8) you have too many irons in the fire > in the smoke
(9) that's a horse of another color > ...of another race
Note that it isn't just any word that is substituted, but one that is related in meaning. Thus the substitution of a word such as 'table' for the word 'up' in (7) or the word 'coat' for the word 'fire' in (8) do not occur. Furthermore, nouns are substituted for nouns, verbs for verbs, and prepositions for prepositions. So even if speakers have never had a class in English grammar they must know unconsciously what grammatical classes these words are in. There are word substitutions in which the two words--intended and spoken--are not related in meaning but are similar in their sounds such as 'persecuted' for 'prosecuted'. The similarity of these words in sound or meaning suggest that we store words in our mental dictionary in semantic classes (according to their related meanings) and also by their sounds (similar to the spelling sequences in a printed dictionary).
The male student who said, "Don't consider this an erection on my part" for the intended "Don't consider this a rejection on my part" while talking to a female student may have produced a 'Freudian slip', but his error also reveals something about the mental representation and processing of what we know about the language we speak.