Why are some metaphors not similes?

Question

This Master Class website says that

A simile is a type of metaphor. All similes are metaphors, but not all metaphors are similes.

1. Is this true? Can anyone cite an official textbook?

2. Please explain why some metaphors AREN'T similes like I'm 10? I never studied linguistics, literature, or literary theory. I read the following posts on ELU: Similes and Metaphors - are similes a subset of metaphors?, The difference between an analogy and a metaphor?, Are analogies and metaphors both forms of comparison, with analogies being analytical thinking and metaphors being creative or lateral thinking?, Is this an analogy or metaphor or what?. But most of these answers are too esoteric for me!

3. Can you answer my question 2 using diagrams or pictures like these?

Source: Analogy vs Metaphor vs Simile on Metamia.com.

Simile vs. Metaphor vs. Analogy: Definitions and Examples by Sean Glatch on Writers.com.

Answer 1

1. It doesn't make sense to say that a definition is true or false. It is clear from the links you've provided that some people use the words metaphor and simile with these meanings. Other people use the words differently, giving them definitions which do not overlap. Neither use is correct or incorrect.

2. Using the definition provided in your first link, a metaphor is a comparison of two different things. A simile is a special kind of metaphor, which compares two different things using the words like or as.
   
   For example: All the world is a stage is a metaphor because it compares the world and a stage. It is not a simile because it does not use the word like or as.
   
   To make the metaphor all the world is a stage into a simile, we would add like: all the world is like a stage

Answer 2

According to dictionaries of literary terms, both similes and metaphors are figures of speech, but similes are usually not considered a subset of metaphors. The difference is that similes make an explicit comparison, whereas metaphors don't. Below are a few sources:

The Penguin Dictionary of Literary Terms and Literary Theory by J. A. Cuddon (third edition. Penguin, 1992):

metaphor (…) A figure of speech in which one thing is described in terms of another. The basic figure in poetry. A comparison is usually implicit; whereas in simile it is explicit.

The following lines from R. S. Thomas's Song at the Year's Turning contain several metaphors but (based on Cuddon's distinction) no similes:

Shelley dreamed it. Now the dream decays.
The props crumble. The familiar ways
Are stale with tears trodden underfoot.
The heart's flower withers at the root.
Bury it, then, in history's sterile dust.
The slow years shall tame your tawny lust.

Cuddon's entry for simile contains the following statements:

It is an explicit comparison (as opposed to the metaphor, where the comparison is implicit) recognizable by the use of the words 'like' or 'as'.

Cuddon gives the following example from Graham Greene's Stamboul Train:

The great blast furnaces of Liège rose along the line like ancient castles burning in a border raid.

The Concise Oxford Dictionary of Literary Terms by Chris Baldick (second edition. Oxford University Press, 2001) contains the following definition of metaphor:

the most important and widespread figure of speech, in which one thing, idea, or action is referred to by a word or expression normally denoting another thing, idea, or action, so as to suggest some common quality shared by the two. In metaphor, this resemblance is assumed as an imaginary identity rather than directly stated as a comparison: referring to a man as that pig, or saying he is a pig is metaphorical, whereas he is like a pig is a simile. (…)

Baldick also gives the following definition of simile:

an explicit comparison between two different things, actions, or feelings, using the words 'as' or 'like', as in Wordsworth's line:

I wandered lonely as a cloud

A very common figure of speech in both prose and verse, simile is more tentative and decorative than metaphor. (…)

Einübung in die Literaturwissenschaft by Harald Fricke and Rüdiger Zymner (Schöningh, 1991) makes the same distinction, as can be seen from the following examples:

• "Du bist ein Esel" ("You are an ass") is given as an example of a metaphor (page 44).
• The following sentences are given as examples of similes (German: "Vergleich", page 52):
  • "Eine Frau ohne Mann ist so komplett wie ein Fisch ohne Fahrrad" ("A woman without a man is as complete as a fish without a bike").
  • "Eine Frau ohne Mann ist wie ein Fisch ohne Fahrrad" ("A woman without a man is like a fish without a bike").

These are three examples of textbooks that make the same distinction between metaphor and simile; based on this, a simile is not a type of metaphor.