the sadness of growing old is part of becoming an individual

--- Thomas Moore, in Care of the Soul: A Guide for Cultivating Depth and Sacredness in Everyday Life, Harper Perennial ppbk 1992, pp. 140-41.

Quote in context:

Aging brings out the flavors of a personality. The individual emerges over time, the way fruit matures and ripens. In the Renaissance view, depression, aging, and individuality all go together: the sadness of growing old is part of becoming an individual. Melancholy thoughts carve out an interior space where wisdom can take up residence.

There are no facts about the future

--- ascribed to risk expert Dr. David T. Hulett on https://www.gristprojectmanagement.us/statistics/there-are-no-facts-about-the-future.html and http://www.johngoodpasture.com/2014/10/there-are-no-facts-about-future.html. I learned it from Ed Thomas, former head of the FCC's Office of Engineering and Technology. It's cited by various people, see e.g. the search results at DuckDuckGo.

And of course, there's always Yogi Berra: “It's tough to make predictions, especially about the future.” 

Jim Gray's Properties of a Research Goal

--- from "What Next? A Few Remaining Problems in Information Technology" (pdf), 1998 Turing Lecture

• Simple to state.
• Not obvious how to do it.
• Clear benefit.
• Progress and solution is testable.
• Can be broken in to smaller steps
• - So that you can see intermediate progress.

... a model ... should yield answers we believe to questions that matter

--- Paul Romer, in his 2015 blog post Speeding-up and Missed Opportunities: Evidence reflecting on the 25th anniversary of the publication of his paper "Endogenous Technological Change" (JPE 1990)

The bar I set for a model is that it should yield answers we believe to questions that matter. For a model of growth, the two questions that matter most are ...

A paradox you live with

--- Sister Gertrude, in Muriel Spark's  The Abbess of Crewe (1974)

From Chapter One, published in The Scotsman, 16 June 2018:

‘Gertrude, my excellent nun, my learned Hun, we have a problem and we don’t know what to do with it.’

‘A problem you solve,’ says Gertrude.

‘Gertrude,’ wheedles the Abbess, ‘we’re in trouble with Rome. The Congregation of Religious has started to probe. They have written delicately to inquire how we reconcile our adherence to the Ancient Rule, which as you know they find suspect, with the laboratory and the courses we are giving the nuns in modern electronics, which, as you know, they find suspect.’

‘That isn’t a problem,’ says Gertrude. ‘It’s a paradox.’

‘Have you time for a very short seminar, Gertrude, on how one treats of a paradox?’

‘A paradox you live with,’ says Gertrude, and hangs up.

The only thing more dangerous than an economist is an amateur economist

--- ascribed to Hal Varian by Ludwig Siegele in "The story of the internet is all about layers" in The Economist's June 2018 Special Report "Fixing the Internet."

However, The Big Apple cites it as "Bentley’s Second Law of Economics" in an entry from July 30, 2010, suggesting that it has been cited in print since at least 2002. It also observes,

“Bentley’s second law” has been accompanied by another law since at least 2002: “Berta’s Fundamental Law of Economic Rents: The only thing more dangerous than an amateur economist is a professional economist.”