What philosophical system did some greek philosophers try to support by arguing that matter is made of particles?

Does this sound good? hey great britain, we need to talk. we’ve been through a lot these past couple of years and i don’t think that i would’ve made it this far without you but, i think it’s time that i tried to. don’t get me wrong, i think you’re a pretty great country! you’re smart, you’re funny, you're rich, and very dominant but, these past couple of years you’ve been a little too dominant. as each year goes by you become more and more self-absorbed. you’re only concerned about your own well-being. for example, when we get into arguments you always raise the taxes on paper and tea prices; it just isn’t fair. not only are you selfish but, you always want your buddies around. it’s not very fun cleaning up after you and your british soldiers (you guys never put the toilet seat down). they’re always popping up unexpected, and you always force me to let them stay for a couple of days. not to mention how irresponsible you are. i don’t like it when you force some of your responsibility on me. it’s not my fault that you got yourself into another war so, why should i have to pay for it? listen, in the beginning, what we had was good but, you and i aren’t so compatible anymore. i think it’s best if i took some time to be independent and think for myself for once. -yours truly, the states