The Marxist intellectual Antonio Gramsci edited a newspaper with this headline in Mussolini's Italy while he struggled against fascism. The saying means that we have a responsibility to be realistic about the terrible political situation in which we find ourselves ("pessimism of the intellect") yet we cannot give up on hoping and struggling for a better world ("optimism of the will"). It's a good motto for life in general, I think. In responding to you I realized I copied it incorrectly and have edited it so it reads correctly now:)