The ancient Greeks had different words for different kinds of love - like ludus (playful love), pragma (longstanding love) and agape (universal love). Sixteen hundred years ago, Augustine argued that the essence of a good life is choosing the right things to love and loving them well.
But over the past several centuries our models of human behaviour have amputated love. Hobbes and other philosophers argued that society is a machine driven by selfishness. Enlightenment philosophers emphasised reason over emotion. Contemporary social science was built on the idea that we're self-interested, calculating creatures.
This philosophical shift has caused unimaginable harm, especially in education.
Education is one of those spheres where the heart is inseparable from the head. If students are going to succeed, they probably need to come from a home where they feel safe and secure, so they aren't paralysed by anxiety and fear. They probably need to have experienced strong attachments so they know how to bond with teachers and parents. They probably need to have been bathed in love so they have some sense of identity, some confidence about their own worth and some sense of agency about their own future.
Even within the classroom, the key fact is the love between a teacher and a student: the teacher's willingness to pour time, attention and care into the student; the student's desire to be worthy of that care and win affection and approval.
For years, schools didn't have to think about love because there were so many other nurturing social institutions. But recently the family has frazzled and community has frayed. Today many students come to school lacking a secure emotional base.
Basically what's happened over the past generation is that we've put enormous effort into improving the academic piece of schooling, but progress has been nil because the students' emotional foundation has been collapsing under our feet. The schools are better than they were, but the gap between the rich and the poor is just as great as it was 20 years ago because the emotional environment is worse.
The good news is that attention is finally turning to the love lives of our students - to the psychic and emotional qualities they bring to the classroom. No one is better at chronicling this shift than Paul Tough, the author of How Children Succeed and now Helping Children Succeed.
In his latest book, he asks how, concretely, we can improve students' non-cognitive skills. ("Non-cognitive skills" is a euphemism social scientists use for those things students get from love and attachment.) Tough notices that many of the teachers who improve their students' character never actually talk about character. They coach them in chess, or enthuse over science. Tough concludes that skills like resilience and self-control are not really skills the way reading is a skill; they are traits imparted by an environment.
The most important educational environment is the one that surrounds a child in the first five years, when the emotional foundations are being engraved. The gap between rich and poor students opens up before age five and stays pretty constant through high school. Despite this, the United States ranks 31st out of 32 developed nations in the portion of public money it spends on early childhood.
Better policy can help. Some of the best programmes help parents do what they are already doing but more consistently - to have "serve and return" interactions with their kids; to practise distanced empathy - to hear their children when they are upset, and to guide them back towards calmness.
Tough reports on research by Professor Roland Fryer at Harvard showing that attempts to pay kids to read more and perform better have been largely ineffective. Students are not motivated by financial incentives. He also reports on research by Associate Professor C. Kirabo Jackson at Northwestern University, which shows that while some teachers are good at raising their students' test scores, other teachers are really good at improving their students' school engagement.
Teachers in the first group are amply rewarded these days, but teachers who motivate their students to show up every day and throw themselves into school life may not even realise how good they are, because emotional engagement is not something we measure and stress.
Teachers are now called upon not only to teach biology but also to create a culture: a culture of caring criticism, so students feel loved while they improve; a culture of belonging, so fragile students feel their work has value. Suddenly, teachers must teach students how to feel about their own feelings; how not to be swallowed up by moments of failure, anger and sadness, but to slow the moment and step outside the emotional spiral.
Many teachers sense that students are more emotionally vulnerable today. Social policy has to find a hundred ways to nurture loving relationships. We have to fortify the heart if we're going to educate the mind.
NEW YORK TIMES