Not too long ago I had a conversation with somebody about good literature. Not specifically what we thought was good literature but what makes good literature. One of the comments that came up - I don't remember who said it, but one of us - was to the effect that people will enjoy a well written book but people will remember a thematically strong book.
Leaving, just for now, whether or not good writing necessitates strong themes let's think about this for a little while. For starters, how do we know what the theme is? Well, that's easy. Because "What are the themes" is the same as asking "What is the book about?" We'll look at one of my favourite novels: "The Shining" by Stephen King.
What's 'The Shining' about?
Well it's about a family who go to stay in a hotel while it's closed for winter because the father needs a job. But the young son is a powerful psychic and the hotel is haunted and it feeds off the child's power to become stronger and terrorizes the family because more ghosts also make it stronger and the hotel, while alive, is not really sentient and just wants to feed and become bigger and stronger like some kind of ghostly psychic animal. But it can't do it on its own and so it manipulates the father who is a recovering alcoholic and has an extreme lack of self-confidence and a desperate need to prove he is still a man, even though he is weak and broken.
No. That's the plot, that's what happens in The Shining but it's not what The Shining is about... Unless you're talking about the 1980 film version in which case it really doesn't go deeper. What you see is what you get.
But what is 'The Shining' about? What is 'The Shining' saying?
'The Shining' is about addiction and recovery. It's about trust in family and how a family can break apart under pressure, especially if there's already a lack of trust. It's about being taken advantage of when you're weak and alone. It's about whether or not you can be strong on your own or if you need others with you. Trust, addiction, family break downs - these are the themes. All the horror, all the scares and the monsters and the ghosts just exist to talk about the themes and say something. And when you get to the end of a book and you look back on it, what stands out is how it felt when you were reading it and those themes came through the story.
I thought 'The Shining' really was a frightening and well written book. But there are plenty of frightening and well written books. 'Endurance' by Joe Konrath was frightening. 'Ring' by Koji Suzuki was pretty scary. 'The Shadow Over Innsmouth' had some creepy moments and all of them (we can debate how good Lovecraft's writing was later) were well written. But why aren't they one of my favourite books and 'The Shining' is? Because the themes in 'The Shining' resonated with me more than the others, I understood them, I related to them and one could argue that the themes were also stronger than in 'Endurance' or 'Ring'. And by stronger I mean more present or at least better said.
Alright, so now we know what a theme is and what they're for. So how do we use them? You're in luck because that's not too hard.
In my experience, themes happen without you noticing. The more you write - especially the more you write of one piece - the more themes are going to slip in unintentionally. So yes, novels will be thematically stronger than short stories as a general rule. Only because there is more time for these things to develop. The reason themes happen like this or that they can happen at all is because people believe things. People have opinions on the world and on society and on relationships. You can't help thinking things about stuff because that is just going to happen as long as you're breathing. A dog can walk past and sniff at your shoes and you will have an opinion about it. When you write, how you see the world and what you believe about the human experience is going to come through in your writing unintentionally. So yay for being human!
But that doesn't mean it's going to be strong or clear. If you think that maybe your writing isn't feeling themey enough, well then maybe that's a good thing to fix. And it's not hard to fix because, like I said, you already have beliefs and opinions and those are the basis for themes.
Look at what you're writing. Think about where the characters are at the beginning and think about where they're going to be at the end (Not sure where that is? You need to go back a few steps, then). Think about the cause of the central conflict. Think about the relationships between characters and how the events of the story impact or change those relationships. Even if your story is about a wizard and his pent monkey fighting space gorillas, you can still reduce the elements of your narrative to the human experience. Now try and reduce those ideas down to a short phrase and be specific. Don't say the theme is love, say the theme is "love in war".
And now you've found your themes and you'll probably find that you've been writing them all along, even if you didn't know it. It's okay for writing to be organic. It should be a little organic and natural. In the same way the planting a grape vine next to a wooden pole lets it grown organically and naturally. Guide your writing.
Once you've found those themes and you know what you're trying to say about the human experience you can begin to alter your story to say more and say louder. Your story will almost certainly be better and more memorable because of it.