We are likely to think that all ideas are things that occur within our mind and are thus subjective. It may be difficult to imagine that there are objective ideas. But there are objective ideas, and it can be understood by examining the meaning of the word “idea.”
The first meaning of “idea” is the content we have in our minds when we are thinking. It includes the sensations and perceptions we have, the images we form, the memories we summon up, and the conceptions or notions that we employ in our thinking.
When the word “idea” is used in this way, all the various items referred to are subjective. My sensations or perceptions are not yours; the images that occur in my dreams or the memories I dwell upon when I reminisce are mine alone; so too are the concepts or notions I have formed as I study a difficult topic.
To call them “subjective” is simply to say that they are private, not public. When I speak of them as mine – my perception, my memory, or my concept – I am saying that the perception, memory, or concept in question belongs to me alone. You can have no access to it, just as you cannot have access to the toothache I am suffering.
In its other meaning, the word “idea” refers to an object that two or more persons can have access to, can focus on, can think about, can discuss.
If we disagree about a decision just handed down by the Supreme Court, we may find ourselves challenging each other’s views about justice. It I ask you for your view of justice, I am asking you to tell me what you think about it, and I am also prepared to tell you what I think about it. The “it” here is justice as an object of thought, both your thought and mine, not justice as a concept in your mind, but not mine.
We each have concepts in our minds – concepts we think with when we think about justice. Furthermore, your concepts and mine are distinct. But that does not prevent both of us from thinking about one and the same object – an object of thought we call “justice”, and sometimes we refer to as “the idea of justice.”
— Six Great Ideas by Mortimer J. Adler