The Ocotillo is not a Cactus – On the Importance of Definitions

Properly defining terms is necessary for understanding.  In a Christian context, it is important to know the meaning of the terms worship, praise, and holy, etc.  Particularly challenging, and thus all the more necessary, is properly defining the term “person” as it relates to the Trinity.  Some of these terms, such as “holy” are used in Scripture and thus meaning can be found in context.  Other terms, such as “Trinity” are not used in Scripture but are concepts developed by Theologians to understand Biblical truths.
In the works of Gordon Clark a strong emphasis is placed on defining terms.   He says “unless one knows the definition, he does not know what he is talking about.”  Yet, even so-called experts in their fields rarely do so.
The following is from the audio lecture: Gordon Clark: Is Christianity a Religion, Part 1, Minutes 7-11
Out West, in Southern Arizona there are cacti.  At this time of year, or in a few weeks from now the desert will not merely blossom like the rose it will outdo the rose for cacti flowers are for more beautiful than the rose. If you go out there at the proper time you’ll see a plant that grows anywhere from 12 to 15 to 20 feet tall.  It’s name is the ocotillo and it has beautiful red blooms on it.
Well, one day I went into a Ranger station and looked at the exhibits there.  And in one of the rooms on the wall there was a painting, an oil painting, of an ocotillo and of course there were ocotillos growing outside. And underneath among some other little bit of information on a placard it says. “The Ocotillo is not a cactus.”  Something an easterner would not have guessed because an ocotillo has thorns you better be careful about touching it.  And it to the unaided eye it looks very much like the other cacti.  As I went to the ranger desk and there was a man there I said, “you have a picture in the other room there, a nice painting of an ocotillo and underneath it says ‘an ocotillo is not a cactus’.
He said “That’s right, an ocotillo is not a cactus.”
I said, “Would you please tell me what is a cactus.  He looked at me “no one has ever asked me that question before.”
I guess not many philosophers have got there before.
He said “I don’t know, but the head Ranger is coming back in a couple minutes, we’ll ask him.”
Well the main ranger came back and so I came up and said “Would you kindly tell me sir, what is a cactus?”
He said, I don’t know, but I’ll look it up for you.  So he got out some of his books and then he gave me a pretty fair statement of what a cactus is.  I later found a better statement, but he gave it a fair try.
You have to define your terms you know.  In particular when you’re studying apologetics you have to known what an ocotillo is, and it isn’t a cactus.
and in another place Clark says:
“The word essence derives from the Latin infinitive esse, which means to be.  The essence of a thing is what the thing is.  It is why the thing is what it is.  What, for example, is the essence of a cactus?  Why is not an ocotillo a cactus?  The answer is that it is the essence of a cactus to have no true leaves, and since ocotillos have true leaves, they are not cacti.  The essentials of a cactus are succulent, absence of true leaves, and three other essentials that I have forgotten.  That is to say, essence means definition.  But unless one knows the definition, he does not know what he is talking about.“