Abhav aur Upasthiti ke beech ki samasya

Mujhe abhi tak to sahi se samajh nahi aaya ki abhav aur upasthiti kya hain. Maine socha tha abhav se upasthiti ka matlab hota hai ki ek cheez present nahi hai, lekin abhi to confusion hai. Kya aap bata sakte hain ki dono mein kya fark hai? Main to sochta hoon ki abhav se upasthiti mein koi jaankari ya praman ki aavashyakata hoti hai, lekin mujhe lagta hai ki iske liye to koi aavashyakata nahi hai. Main aisey students hoon jo abhi tak sahi se samajh nahin paane mein lag rahe hain. Aap log kya bolenge?