It is wrong to suppose that words can be used only to talk about their cus-tomary references. We have seen how it is possible to talk about words, and the sense of words, without talking about the reference of those words.
Concerning this point, Frege states the following:
The reference and sense of a sign are to be distinguished from the associated idea. If the reference of a sign is an object perceivable by the senses, my idea of it is an
in-ternal image, arising from memories of sense impressions which I have had and acts, both internal and external, which I have performed. Such an idea is often saturated with feeling; the clarity of its separate parts varies and oscillates. The same sense is not always connected, even in the same man, with the same idea. The idea is subjec-tive: one man’s idea is not that of another. There result, as a matter of course, a vari-ety of differences in the ideas associated with the same sense. A painter, a horseman, and a zoologist will probably connect different ideas with the name ‘Bucephalus’.
This constitutes an essential distinction between the idea and the sign’s sense, which may be the common property of many and therefore is not a part of a mode of the individual mind. For one can hardly deny that mankind has a common store of thoughts which is transmitted from one generation to another.13
In this passage, Frege sharply distinguishes between ideas present in peo-ple’s minds and the sense and reference of words. To reiterate a point made above, Frege does not think that the ideas present in people’s minds have anything essentially to do with sense and reference at all. A psychological idea may be necessary for a human being to grasp a sense, but that does not mean that the sense is the same thing as the idea.
First, depending on who you are, a certain word will bring different ideas to mind. For example, an equestrian will have a different idea come to mind when he hears the word “horse” uttered than when a zoologist hears the same word. Frege thinks that the sense of the word “horse” is the same for both of those individuals—the only difference lies in the different men-tal associations each person has for that word. Furthermore, over time an individual can come to have different emotional associations with the same word. In that case, Frege does not think that the sense changes; rather, the mental associations do. Mental associations can change, but the sense will stay the same.
The second reason he gives for making this distinction is that mankind acquires a stock of knowledge, a series of propositions we believe, and we pass those propositions on from generation to generation. Therefore, in a nonpsychological sense, the same thought (or proposition) is transmit-ted from one generation to another. This process concerns something that transcends the individual persons and the minds that are responsible for the transmitting. For example, consider Isaac Newton in the eighteenth century with various thoughts going through his mind. Suddenly, he states that gravity obeys the inverse square law and writes it in his Principia. After this event, everyone who reads Principia acquires that thought, down the ages, until the present day. Knowing such a thing is different from knowing
Newton’s subjective, psychological ideas. Hence, when Frege speaks of thoughts he refers to something that is objective and transcends time—
a thought is the objective unchanging sense of a sentence. Thoughts, in Frege’s use, are abstract entities.
Ideas are not the same as senses; rather, they are things that perish when the mind that has them perishes. Ideas are not really shared by people.
Senses, however, are shared by people and do not perish with an individual mind. For Frege, senses have the same objectivity and mental independence as references. The sense of the word “gravity” existed back in Newton’s time and we grasp that same sense now. Therefore, many subjective ideas can correspond to the same objective sense. Frege’s general purpose in arguing for senses to be objective is to show the objective basis for mathematics and science in general.
It is important to note that ideas can also be objects of reference. In normal speech, people do not typically talk about ideas. People have ideas all the time, but they do not usually refer to them. For example, if someone says, “It’s raining outside,” she is not saying anything about ideas at all. If she were talking about ideas, she would say something like, “My idea that it is raining outside is well founded.” Just as senses and words can be the objects of reference, so too can ideas be the object of reference.
Frege constructs a complete picture for organizing all of these aspects of language by forming a system of levels—words, ideas, senses, and refer-ences. He illustrates his leveled system with an analogy:
The reference of a proper name is the object itself which we designate by its means;
the idea, which we have in that case, is wholly subjective; in between lies the sense, which is indeed no longer subjective like the idea, but is yet not the object itself. The following analogy will perhaps clarify these relationships. Somebody observes the Moon through a telescope. I compare the Moon itself to the reference; it is the object of the observation, mediated by the real image projected by the object on the glass in the interior of the telescope, and by the retinal image of the observer. The former I compare to the sense, the latter is like the idea or experience. The optical image in the telescope is indeed one-sided and dependent upon the standpoint of observa-tion; but it is still objective, inasmuch as it can be used by several observers. At any rate it could be arranged for several to use it simultaneously. But each one would have his own retinal image. On account of the diverse shapes of the observer’s eyes, even a geometrical congruence could hardly be achieved, and an actual coincidence would be out of the question. This analogy might be developed still further, by as-suming A’s retinal image made visible to B; or A might also see his own retinal image in a mirror. In this way we might perhaps show how an idea can itself be taken as an
object, but as such is not for the observer what it directly is for the person having the idea. But to pursue this would take us too far afield.14
There is the telescope, the object observed through the telescope, the opti-cal image on the lens of the telescope, and the retinal image on the eye of the observer. The retinal image is also an optical pattern that is projected through the lens of the eye and passes on to the retina. There appear to be three levels: the object out there, the optical image on the lens, and the retinal image. Frege compares the optical image to the sense, and the idea to the retinal image. The retinal image is different for each individual per-son who looks through the telescope because we all have different retinal structures. However, he thinks that the optical image is the same, even though people observe it with different retinas. Therefore, the sense is an objective thing in the same way that the optical image is an objective thing, and different from the retinal image, which is subjective and depends on an individual’s physiological makeup.