If I had to pick one word that nails that slow, deep kind of thinking, I'd reach for 'ruminate'. To me it carries the image of turning an idea over and over in your head — like chewing on a tune until all the notes make sense. I use it whenever I'm not just skimming possibilities but really letting something settle: past decisions, bittersweet memories, or a stubborn plot twist in a story that won’t leave me alone. 'Ruminate' sounds patient and a little melancholy, which is perfect for wistful or weighty reflection.
There are other good choices depending on the shade you want. 'Contemplate' feels more spacious and slightly formal — great for quiet, almost reverent consideration. 'Deliberate' brings to mind weighing pros and cons, often with a group or the need to decide. 'Meditate on' leans spiritual or disciplined, while 'brood' suggests darker, worry-driven thinking. I tend to default to 'ruminate' when I want to communicate depth without sounding clinical; it fits journal entries, slow scenes in novels, or late-night conversations with friends. Honestly, I find the sound of the word helps set the mood: slow, thoughtful, a little bittersweet. That's why 'ruminate' is my go-to for deep reflection.