But it's merely a phrase, riddled with ambiguity and fraught with subjective reinterpretation. In short, one cannot easily build a policy statement based on a fuzzy phrase.

Rest easy gentle readers. There's a better way. And we might already be there. (See "A Brave New World for 401k Plans Sponsors without Conflict-of-Interest Fees," FiduciaryNews.com, April 4, 2016)

Traditionally – and this goes back to the original Magna Carta – a trustee has been bound by a fiduciary duty to the beneficiaries of the assets entrusted to the trustee. One of the primary rules has always been "no self-dealing."

Read: A motto for fiduciaries

This doesn't mean the trustee cannot obtain a fee. It merely states that a trustee cannot engage in a transaction using beneficiary assets that produces compensation for the trustee.

For example, a trustee cannot buy a car on behalf of a beneficiary if that car is the property of the trustee. Additionally, a trustee cannot purchase a car on behalf of beneficiary if the owner of the car promises to give the trustee a commission as a result of the sale.

It's pretty simple. A trustee can get paid, but a trustee cannot engage in a transaction that causes the trustee to get paid.

It's all about the numbers, specifically, the numbers that represent an accounting of fees. It's simple because numbers are measurable. Unlike a slogan like "best interests," numbers present no opportunity for wiggle room. They either are or they aren't.

With this in mind, we can quickly create a fiduciary rule that is both easy to understand and easy to enforce. We can merely borrow from the same traditional understanding upon which trust law (the father of fiduciary law) has been founded: Let there be no self-dealing transactions.

In the world of advice, there are three obvious self-dealing transactions: Commissions, 12b-1 fees, and revenue sharing.

Rather than rely on the expression "best interests," a better fiduciary rule would, in black and white terms, prohibit anyone offering advice from getting paid via commissions, 12b-1 fees, and revenues.

Note, this does not prohibit commissions, 12b-1 fees, and revenues. It only bans advisers from obtaining fees through these methods. Brokers, on the other hand, can (and the capital markets will argue they must) continue to rely on these fee methods.

It's no big deal. As the article cited above demonstrates, there's a long-standing business model that has been following this measurable fiduciary rule for decades.

Numbers are easier than words. Numbers are measurable. Words always leave something out. Of course, if we make numbers-based rules, how would politicians, lawyers, and other sophists earn their living?