Google co-founder Sergey Brin has warned that the current boom in artificial intelligence has created a “technology renaissance” that contains many potential threats. Writing in the company’s annual Founders’ Letter, published Friday, the Alphabet president struck a note of caution. “The new spring in artificial intelligence is the most significant development in computing in my lifetime,” writes Brin. “Every month, there are stunning new applications and transformative new techniques.” But, he adds, “such powerful tools also bring with them new questions and responsibilities.”

Brin starts his letter by quoting the opening lines of Charles Dickens’ A Tale of Two Cities — “It was the best of times, it was the worst of times.” He notes how computing power has exploded since Google was founded in 1998, and how, at that time, the technique which now forms the backbone of contemporary AI, neural networks, was just “a forgotten footnote in computer science.”

The revolution in machine learning in the past decade has changed that, and Brin lists some of the many ways AI is used to power Alphabet services and companies. It analyzes images in Google Photos; translates over 100 languages in Google translate; powers the navigation systems in Waymo’s self-driving cars; and even helps diagnose disease and discover new planetary systems. “In this sense, we are truly in a technology renaissance, an exciting time where we can see applications across nearly every segment of modern society,” writes Brin.

But, he says, AI poses a number of problems too, “from the fears of sci-fi style sentience to the more near-term questions such as validating the performance of self-driving cars.” Brin says Alphabet is giving “serious thought” to a number of these issues, including how AI will affect employment; the challenges of making unbiased and transparent algorithms; and the fears that this technology will be used to “manipulate people.” (This is most likely a reference to recent discussions of AI-generated fake news.)

Notably, though, Brin does not mention one controversial use of AI that is particularly relevant to Alphabet: military applications. Earlier this year, it was revealed that Google was helping the Pentagon deploy machine learning tools to analyze video surveillance footage from drones. The company has said the tech is for “non-offensive uses only,” but thousands of Google employees have demanded that the company withdraw from the project.

Brin’s letter also comes at a time when Silicon Valley firms are facing more scrutiny than ever, with governments and the public come to terms with their huge size and wealth. Last week’s tech earning reports will have done nothing to dispel such fears, as Alphabet reported a 26 percent increase in revenues to $31.1 billion, and Amazon’s earnings jumped up 43 percent year on year, with first quarter sales of $51 billion.

Fear of AI destroying jobs and manipulating politics will likely be a part of any future discussions of the need to curb Big Tech’s power and influence. This, as much as anything else, likely explains Brin’s somber tone. He closes the letter by saying: “While I am optimistic about the potential to bring technology to bear on the greatest problems in the world, we are on a path that we must tread with deep responsibility, care, and humility.” The unspoken message? There’s no turning back.