Some of the best business advice is more than 2,000 years old. Many entrepreneurs embrace the teachings of Greek philosophy to help shape and guide their life and careers.

One school of philosophy, called stoicism, is especially popular among luminaries like Bill Gates, Warren Buffet, and Conde Nast International CEO Jonathan Newhouse. One way Gates and Buffett practice stoicism is how simply they live despite being worth billions. Newhouse has said that stoicism has taught him how to better accept what he can't control.

Stoicism was founded in the early 3rd century BC and revolves around three basic ideas: How life is brief and the world is unpredictable; how to be steadfast, strong, and in control of yourself; and how dissatisfaction comes from impulsive reflexes rather than logic.

There are several famous stoic philosophers with many individual teachings, but here are four that can help you confront the ongoing challenges in business.

1. Never let wealth distort your values.

"For the wise man does not consider himself unworthy of any gifts from Fortune's hands: he does not love wealth but he would rather have it; he does not admit into his heart but into his home; and what wealth is his he does not reject but keeps, wishing it to supply greater scope for him to practice his virtue."--Seneca

Stoicism teaches that there is nothing wrong with achieving monetary success; however, you should never compromise your principles in its pursuit. True stoics prefer good fortune, but they don't require it.  This can be a constant struggle, especially when faced with tough decisions regarding profit over principles, but if you always bow to wealth, you will soon become trapped.

I once had a large client who was always horrible to my staff. I discussed it with the client, but to no avail. I had to let them go in order to protect my team and my values, and I had faith that new revenue would come along. The result was that my staff was more loyal than ever because they knew what a huge financial risk I took.  

2. Build a village.

"The key is to keep company only with people who uplift you, whose presence calls forth your best."-- Epictetus

You are only as strong as the people around you. I always encourage my staff to offer suggestions and engage in brainstorming sessions when we need to find a new path forward. The people closest to the day-to-day business often provide the clearest insight, which helps me see different perspectives and make better decisions.

You can control whom you interact with, so follow this stoicism advice and build a strong personal and professional coalition--hire people with positive energy and create a circle of friends from different backgrounds for engaging conversations.

3. Embrace rejection as insight.

"Everything we hear is an opinion, not a fact."--Marcus Aurelius

As entrepreneurs, you hear far more "No's" than "Yes's." This does not mean your ideas should be abandoned. Instead, use feedback as valuable insight. I always look at the "No's" as an opportunity. What can I do to turn a "No" into a "Yes"? There is often a solution if you are open to constructive criticism.

4. Never do anything out of habit.

"So in the majority of other things, we address circumstances not in accordance with the right assumptions, but mostly by following wretched habit." -- Gaius Musonius Rufus

Routine and habits can chip away at efficiency. The stoic lesson here is to always think about how you can improve your daily life. Review your regular work routines and project procedures. What can you change to improve productivity? When I get stuck in around-the-clock work, I break the cycle by scheduling more personal and family time. This not only breaks up my routine work, but clears my mind do I can return to work with greater focus.

Sometimes the most complex issues in business and life can be solved by the simplest of means. The Greek philosophy of stoicism teaches you the basic truths about life. Embracing this wisdom can provide the guidance you need to be a better person at work and home.