Answer:
The three core principles of liberals were equality before the law, government based on the consent of the people, and individual freedom to engage in economic activity without governmental interference.
What are the core principles of Liberalism? Are rule of law, equality and collective security liberal values?
There is a line of thought, especially in the USA, that Conservatism and Liberalism are some sort of mirror images of each other. For instance, an often-voiced principle of US Conservatism is a desire of small government. And many conservatives appear to assume that Liberals must be for big government. In fact, US Liberals tend to believe that government can and should have a positive role in the US and in the World. That does mean that Liberals will propose ways in which the government can have a positive role, and will protect against attempts to eliminate those roles, especially when the reason amounts to “government is too big.”
It’s also true that over the past several decades, the US government has grown increasingly conservative. President Richard Nixon was seen as a staunch conservative, but would be completely rejected by the current Republican party. President Ronald Reagan would be a moderate by today’s Republicans.
Meanwhile, at least on social issues, the American people have, over those same several decades, become increasingly liberal. Note the way most Americans have become more accepting of gays and lesbians, of mixed-race marriages, of divorce and re-marriage, of unwed couples.
These two trends combine, along with some inherent traits, to make Liberals less dogmatic than the typical Conservatives in the US. They are more willing to join forces with people who may differ on some come principles, so long as they agree on most.
All that is to say, the core principles of US Liberals may be a bit more difficult to define than you might expect. However, here are some that I can identify: (Note: any use of “Liberals” or “Conservatives” below are generalizations. There are exceptions in either case.)
Liberals believe that all people have great intrinsic worth, by virtue of being people. Liberals recoil at the idea that some people are inherently “better” than other people. (Please note that isn’t to say that some people are better at some things than others).
As mentioned above, Liberals have a more positive attitude about government than Conservatives. Liberals take seriously Lincoln’s idea of government “of the people, by the people, and for the people”.
Liberals support a restricted free market. I like the idea of “as free as possible, as restricted as necessary.” In particular, Liberals recognise that the wealth and power of some corporations and individuals can be destructive to free markets and oppressive of free people.
Liberals understand society as a cooperative endeavor. Society, with the assistance (and often leadership) of government should work for the common good. As in all cooperative endeavors, that means that individuals must sometimes sacrifice for the good of the whole.
Liberals respect expertise, and seek well-qualified experts as advisors and as leaders. The idea that “my uniformed opinion is as good as your expertise” is ridiculous to Liberals. This means that Liberals tend to believe scientists in their area of expertise, especially when there is near consensus. Liberals also would not want to watch an NFL game where the players were all chosen at random from the fans in attendance.
I am sure that there are more that others can mention. I’m also sure that some Liberals will disagree with me. That’s ok. Because Liberals also support the idea of exchanging ideas and disagreeing respectfully. Although we don’t always live up to that ideal.