Historian: The International Space Station is a very big mission in many ways. The mission is international. The mission has mountains of parts — there might not be a full list of all the parts. The missions takes teams of teams of teams sharing the work — there might not be a full list of all of the teams.
Physicist: There are things we want to learn which we can not learn except via the ISS mission. If we want to wander beyond our Moon, then we will need knowledge which we can only gather via the ISS mission.
Historian: The ISS mission contributes to sustaining Earth by showing us views of Earth by ISS mission astronauts. The ISS mission astronauts contribute to sustaining Earth by inspiring young persons to pursue science.
Physicist: The ISS mission — and other of our missions off Earth — inspire us by examples. The ISS mission — and other of our missions off Earth — change the meaning of "humanity" and, we can hope, help make us better. The NASA and ESA ISS mission pages give us great substance to fuel our hopes.
NASA ISS mission page [link]
ESA ISS mission page [link]
Tour ISS by ESA [link]
Building the International Space Station [link]
ISS 2015 highlights [link]