Truth vs Fantasy
On face value, the philosophical conflict (the theme) in Inception is about whether you should live in a comfortable illusion (Fantasy), or whether you should keep a grip on the real world, even if it is unpleasant (Reality). This conflict can also be phrased as Truth vs Fantasy.
I think Inception has one of the greatest concepts to support its philosophical conflict, out of any movie ever. What better way to externalize living a fantasy, than characters actually being able to live in a dream.
The character Mal Cobb believes that you should embrace fantasy. That dreams can feel just as meaningful as waking life.
The protagonist Dom Cobb struggles with what he should believe.
The movie invites you to ask yourself:
What assumptions about yourself have you never examined?